Message ID | 20240812101555.3558589-7-mark.rutland@arm.com (mailing list archive) |
---|---|
State | New, archived |
Headers | show |
Series | Cleanup initialization | expand |
On Mon, 12 Aug 2024 11:15:51 +0100 Mark Rutland <mark.rutland@arm.com> wrote: Hi Mark, > When the boot-wrapper is entered at Seculre PL1 it will enter the kernel Secure > via an exception return, ERET, and when entered at Hyp it will branch to > the kernel directly. This is an artifact of the way the boot-wrapper was > originally written in assembly, and it would be preferable to always > enter the kernel via an exception return so that PSTATE is always > initialized to a known-good value. > > Rework jump_kernel() to always enter the kernel via ERET, matching the > stype of the AArch64 version of jump_kernel() type > > Signed-off-by: Mark Rutland <mark.rutland@arm.com> > Acked-by: Marc Zyngier <maz@kernel.org> > Cc: Akos Denke <akos.denke@arm.com> > Cc: Andre Przywara <andre.przywara@arm.com> > Cc: Luca Fancellu <luca.fancellu@arm.com> > --- > arch/aarch32/boot.S | 48 +++++++++++++++++++++++---------------------- > 1 file changed, 25 insertions(+), 23 deletions(-) > > diff --git a/arch/aarch32/boot.S b/arch/aarch32/boot.S > index f21f89a..78d19a0 100644 > --- a/arch/aarch32/boot.S > +++ b/arch/aarch32/boot.S > @@ -76,10 +76,6 @@ reset_at_hyp: > > bl setup_stack > > - mov r0, #1 > - ldr r1, =flag_no_el3 > - str r0, [r1] > - > bl cpu_init_bootwrapper > > bl cpu_init_arch > @@ -96,9 +92,10 @@ err_invalid_id: > * r1-r3, sp[0]: kernel arguments > */ > ASM_FUNC(jump_kernel) > - sub sp, #4 @ Ignore fourth argument Can we maybe keep the comment, to avoid confusion? The comment above explicitly mentions sp[0], but we never use it. > - push {r0 - r3} > - mov r5, sp > + mov r4, r0 > + mov r5, r1 > + mov r6, r2 > + mov r7, r3 > > ldr r0, =HSCTLR_KERNEL > mcr p15, 4, r0, c1, c0, 0 @ HSCTLR > @@ -111,23 +108,28 @@ ASM_FUNC(jump_kernel) > bl find_logical_id > bl setup_stack > > - ldr lr, [r5], #4 > - ldm r5, {r0 - r2} > - > - ldr r4, =flag_no_el3 > - ldr r4, [r4] > - cmp r4, #1 > - bxeq lr @ no EL3 > + mov r0, r5 > + mov r1, r6 > + mov r2, r7 > + ldr r3, =SPSR_KERNEL > > - ldr r4, =SPSR_KERNEL > /* Return in thumb2 mode when bit 0 of address is 1 */ > - tst lr, #1 > - orrne r4, #PSR_T > + tst r4, #1 > + orrne r3, #PSR_T > + > + mrs r5, cpsr > + and r5, #PSR_MODE_MASK > + cmp r5, #PSR_MON > + beq eret_at_mon > + cmp r5, #PSR_HYP > + beq eret_at_hyp > + b . > > - msr spsr_cxf, r4 > +eret_at_mon: > + mov lr, r4 > + msr spsr_cxf, r3 > movs pc, lr Maybe that's just a wording confusion between "exception return instruction" and "eret", but both the commit message and the label promise an eret, and we have a "movs pc" here. Reading "B9.1 General restrictions on system instructions" in the ARMv7 ARM I don't immediately see why an eret wouldn't be possible here. If there is a restriction I missed, I guess either a comment here or in the commit message would be helpful. > - > - .section .data > - .align 2 > -flag_no_el3: > - .long 0 > +eret_at_hyp: > + msr elr_hyp, r4 > + msr spsr_cxf, r3 Shouldn't that be spsr_hyp? Cheers, Andre > + eret
On Mon, Aug 19, 2024 at 06:22:41PM +0100, Andre Przywara wrote: > On Mon, 12 Aug 2024 11:15:51 +0100 > Mark Rutland <mark.rutland@arm.com> wrote: > > Hi Mark, > > > When the boot-wrapper is entered at Seculre PL1 it will enter the kernel > > Secure > > > via an exception return, ERET, and when entered at Hyp it will branch to FWIW, that "ERET, " here was meant to go too (which I think addresses a later concern below). I had taken the commit message wording from the early AArch64 commit and adjusted it to suit AArch32, but clearly I had done that in a rush and madea number of mistakes. > > the kernel directly. This is an artifact of the way the boot-wrapper was > > originally written in assembly, and it would be preferable to always > > enter the kernel via an exception return so that PSTATE is always > > initialized to a known-good value. > > > > Rework jump_kernel() to always enter the kernel via ERET, matching the > > stype of the AArch64 version of jump_kernel() > > type > > > > > Signed-off-by: Mark Rutland <mark.rutland@arm.com> > > Acked-by: Marc Zyngier <maz@kernel.org> > > Cc: Akos Denke <akos.denke@arm.com> > > Cc: Andre Przywara <andre.przywara@arm.com> > > Cc: Luca Fancellu <luca.fancellu@arm.com> > > --- > > arch/aarch32/boot.S | 48 +++++++++++++++++++++++---------------------- > > 1 file changed, 25 insertions(+), 23 deletions(-) > > > > diff --git a/arch/aarch32/boot.S b/arch/aarch32/boot.S > > index f21f89a..78d19a0 100644 > > --- a/arch/aarch32/boot.S > > +++ b/arch/aarch32/boot.S > > @@ -76,10 +76,6 @@ reset_at_hyp: > > > > bl setup_stack > > > > - mov r0, #1 > > - ldr r1, =flag_no_el3 > > - str r0, [r1] > > - > > bl cpu_init_bootwrapper > > > > bl cpu_init_arch > > @@ -96,9 +92,10 @@ err_invalid_id: > > * r1-r3, sp[0]: kernel arguments > > */ > > ASM_FUNC(jump_kernel) > > - sub sp, #4 @ Ignore fourth argument > > Can we maybe keep the comment, to avoid confusion? The comment above > explicitly mentions sp[0], but we never use it. > > > - push {r0 - r3} > > - mov r5, sp > > + mov r4, r0 > > + mov r5, r1 > > + mov r6, r2 > > + mov r7, r3 > > > > ldr r0, =HSCTLR_KERNEL > > mcr p15, 4, r0, c1, c0, 0 @ HSCTLR > > @@ -111,23 +108,28 @@ ASM_FUNC(jump_kernel) > > bl find_logical_id > > bl setup_stack > > > > - ldr lr, [r5], #4 > > - ldm r5, {r0 - r2} > > - > > - ldr r4, =flag_no_el3 > > - ldr r4, [r4] > > - cmp r4, #1 > > - bxeq lr @ no EL3 > > + mov r0, r5 > > + mov r1, r6 > > + mov r2, r7 > > + ldr r3, =SPSR_KERNEL > > > > - ldr r4, =SPSR_KERNEL > > /* Return in thumb2 mode when bit 0 of address is 1 */ > > - tst lr, #1 > > - orrne r4, #PSR_T > > + tst r4, #1 > > + orrne r3, #PSR_T > > + > > + mrs r5, cpsr > > + and r5, #PSR_MODE_MASK > > + cmp r5, #PSR_MON > > + beq eret_at_mon > > + cmp r5, #PSR_HYP > > + beq eret_at_hyp > > + b . > > > > - msr spsr_cxf, r4 > > +eret_at_mon: > > + mov lr, r4 > > + msr spsr_cxf, r3 > > movs pc, lr > > Maybe that's just a wording confusion between "exception return > instruction" and "eret", but both the commit message and the label > promise an eret, and we have a "movs pc" here. Wording-wise, "ERET" was spurious, and the commit message was inteneded to say "via an exception return", with the "movs pc, lr" being the exception return. > Reading "B9.1 General restrictions on system instructions" in the ARMv7 ARM > I don't immediately see why an eret wouldn't be possible here. > > If there is a restriction I missed, I guess either a comment here or in > the commit message would be helpful. We can use ERET here; IIRC that was added in the ARMv7 virtualization extensions, but the boot-wrapper requires that and really it's ARMv8+ anyway. I had opted to stick with "movs pc, lr" because it was a (trivially) smaller change, and kept the cases distinct, but I'm happy to use ERET. However, beware that in AArch32 ERET is a bit odd: in Hyp mode takes the return address from ELR_HYP, while in all other modes it takes it from the LR (as only hyp has an ELR). > > - > > - .section .data > > - .align 2 > > -flag_no_el3: > > - .long 0 > > +eret_at_hyp: > > + msr elr_hyp, r4 > > + msr spsr_cxf, r3 > > Shouldn't that be spsr_hyp? It can be, but doesn't need to be. This is the SPSR_<fields> encoding, which writes to the SPSR for owned by the active mode, though it skips bits<23:16>, which we probably should initialise. If I change that all to: | eret_at_mon: | mov lr, r4 | msr spsr_mon, r3 | eret | eret_at_hyp: | msr elr_hyp, r4 | msr spsr_hyp, r3 | ... do you think that's clear enough, or do you think we need a comment about the "LR" vs "ELR_HYP" distinction? Mark.
On Mon, Aug 19, 2024 at 06:22:41PM +0100, Andre Przywara wrote: > On Mon, 12 Aug 2024 11:15:51 +0100 > Mark Rutland <mark.rutland@arm.com> wrote: > > Rework jump_kernel() to always enter the kernel via ERET, matching the > > stype of the AArch64 version of jump_kernel() > > type That's actually meant to be "style", and "ERET" should be "an exception return", i.e. | Rework jump_kernel() to always enter the kernel via an exception | return, matching the style of the AArch64 version of jump_kernel() Mark.
On Tue, 20 Aug 2024 12:47:59 +0100 Mark Rutland <mark.rutland@arm.com> wrote: > On Mon, Aug 19, 2024 at 06:22:41PM +0100, Andre Przywara wrote: > > On Mon, 12 Aug 2024 11:15:51 +0100 > > Mark Rutland <mark.rutland@arm.com> wrote: > > > Rework jump_kernel() to always enter the kernel via ERET, matching the > > > stype of the AArch64 version of jump_kernel() > > > > type > > That's actually meant to be "style", and "ERET" should be "an exception > return", i.e. Ah, but three tries is not bad for a Wordle ;-) Cheers, Andre > > | Rework jump_kernel() to always enter the kernel via an exception > | return, matching the style of the AArch64 version of jump_kernel() > > Mark.
On Tue, 20 Aug 2024 12:43:18 +0100 Mark Rutland <mark.rutland@arm.com> wrote: Hi, > On Mon, Aug 19, 2024 at 06:22:41PM +0100, Andre Przywara wrote: > > On Mon, 12 Aug 2024 11:15:51 +0100 > > Mark Rutland <mark.rutland@arm.com> wrote: > > > > Hi Mark, > > > > > When the boot-wrapper is entered at Seculre PL1 it will enter the kernel > > > > Secure > > > > > via an exception return, ERET, and when entered at Hyp it will branch to > > FWIW, that "ERET, " here was meant to go too (which I think addresses a > later concern below). I had taken the commit message wording from the > early AArch64 commit and adjusted it to suit AArch32, but clearly I had > done that in a rush and madea number of mistakes. Yeah, no worries, I was just getting a bit confused, especially when I learned more about the subtleties of exception returns on ARMv7 than I ever wanted to know yesterday. > > > the kernel directly. This is an artifact of the way the boot-wrapper was > > > originally written in assembly, and it would be preferable to always > > > enter the kernel via an exception return so that PSTATE is always > > > initialized to a known-good value. > > > > > > Rework jump_kernel() to always enter the kernel via ERET, matching the > > > stype of the AArch64 version of jump_kernel() > > > > type > > > > > > > > Signed-off-by: Mark Rutland <mark.rutland@arm.com> > > > Acked-by: Marc Zyngier <maz@kernel.org> > > > Cc: Akos Denke <akos.denke@arm.com> > > > Cc: Andre Przywara <andre.przywara@arm.com> > > > Cc: Luca Fancellu <luca.fancellu@arm.com> > > > --- > > > arch/aarch32/boot.S | 48 +++++++++++++++++++++++---------------------- > > > 1 file changed, 25 insertions(+), 23 deletions(-) > > > > > > diff --git a/arch/aarch32/boot.S b/arch/aarch32/boot.S > > > index f21f89a..78d19a0 100644 > > > --- a/arch/aarch32/boot.S > > > +++ b/arch/aarch32/boot.S > > > @@ -76,10 +76,6 @@ reset_at_hyp: > > > > > > bl setup_stack > > > > > > - mov r0, #1 > > > - ldr r1, =flag_no_el3 > > > - str r0, [r1] > > > - > > > bl cpu_init_bootwrapper > > > > > > bl cpu_init_arch > > > @@ -96,9 +92,10 @@ err_invalid_id: > > > * r1-r3, sp[0]: kernel arguments > > > */ > > > ASM_FUNC(jump_kernel) > > > - sub sp, #4 @ Ignore fourth argument > > > > Can we maybe keep the comment, to avoid confusion? The comment above > > explicitly mentions sp[0], but we never use it. > > > > > - push {r0 - r3} > > > - mov r5, sp > > > + mov r4, r0 > > > + mov r5, r1 > > > + mov r6, r2 > > > + mov r7, r3 > > > > > > ldr r0, =HSCTLR_KERNEL > > > mcr p15, 4, r0, c1, c0, 0 @ HSCTLR > > > @@ -111,23 +108,28 @@ ASM_FUNC(jump_kernel) > > > bl find_logical_id > > > bl setup_stack > > > > > > - ldr lr, [r5], #4 > > > - ldm r5, {r0 - r2} > > > - > > > - ldr r4, =flag_no_el3 > > > - ldr r4, [r4] > > > - cmp r4, #1 > > > - bxeq lr @ no EL3 > > > + mov r0, r5 > > > + mov r1, r6 > > > + mov r2, r7 > > > + ldr r3, =SPSR_KERNEL > > > > > > - ldr r4, =SPSR_KERNEL > > > /* Return in thumb2 mode when bit 0 of address is 1 */ > > > - tst lr, #1 > > > - orrne r4, #PSR_T > > > + tst r4, #1 > > > + orrne r3, #PSR_T > > > + > > > + mrs r5, cpsr > > > + and r5, #PSR_MODE_MASK > > > + cmp r5, #PSR_MON > > > + beq eret_at_mon > > > + cmp r5, #PSR_HYP > > > + beq eret_at_hyp > > > + b . > > > > > > - msr spsr_cxf, r4 > > > +eret_at_mon: > > > + mov lr, r4 > > > + msr spsr_cxf, r3 > > > movs pc, lr > > > > Maybe that's just a wording confusion between "exception return > > instruction" and "eret", but both the commit message and the label > > promise an eret, and we have a "movs pc" here. > > Wording-wise, "ERET" was spurious, and the commit message was inteneded > to say "via an exception return", with the "movs pc, lr" being the > exception return. Ah thanks, that was one of the two possibilities I considered. > > Reading "B9.1 General restrictions on system instructions" in the ARMv7 ARM > > I don't immediately see why an eret wouldn't be possible here. > > > > If there is a restriction I missed, I guess either a comment here or in > > the commit message would be helpful. > > We can use ERET here; IIRC that was added in the ARMv7 virtualization > extensions, but the boot-wrapper requires that and really it's ARMv8+ Is that so? I mean in all practicality we will indeed use the bootwrapper on ARMv8 only these days, but I don't think we need to artificially limit this. Also I consider the boot-wrapper one of the more reliable sources for ARMv7 boot code, so not sure we should drop this aspect. There is one ARMv7 compile time check, to avoid "sevl", so we have some support, at least. > anyway. I had opted to stick with "movs pc, lr" because it was a > (trivially) smaller change, and kept the cases distinct, but I'm happy > to use ERET. > > However, beware that in AArch32 ERET is a bit odd: in Hyp mode takes the > return address from ELR_HYP, while in all other modes it takes it from > the LR (as only hyp has an ELR). Yeah, I saw this yesterday, and am even more grateful for the ARMv8 exception model now ;-) So I am fine with "movs pc, lr", if that's the more canonical way on 32-bit/ARMv7. On the other hand your revised sequence below looks intriguingly simple ... > > > > - > > > - .section .data > > > - .align 2 > > > -flag_no_el3: > > > - .long 0 > > > +eret_at_hyp: > > > + msr elr_hyp, r4 > > > + msr spsr_cxf, r3 > > > > Shouldn't that be spsr_hyp? > > It can be, but doesn't need to be. This is the SPSR_<fields> encoding, So I didn't know about this until yesterday, and it's not easy to find, since it seems not to be mentioned as such in the ARM ARM (at least not "cxf"). binutils seems to disassemble this to SPSR_fxc, but I guess we should indeed move to SPSR_fsxc (if we keep this at all). > which writes to the SPSR for owned by the active mode, though it skips > bits<23:16>, which we probably should initialise. > > If I change that all to: > > | eret_at_mon: > | mov lr, r4 > | msr spsr_mon, r3 > | eret > | eret_at_hyp: > | msr elr_hyp, r4 > | msr spsr_hyp, r3 > | > > ... do you think that's clear enough, or do you think we need a comment > about the "LR" vs "ELR_HYP" distinction? Oh, that certainly looks the clearest, but indeed a comment on LR vs. ELR situation looks indicated. Thanks, Andre > > Mark.
On Tue, Aug 20, 2024 at 01:59:44PM +0100, Andre Przywara wrote: > On Tue, 20 Aug 2024 12:43:18 +0100 > Mark Rutland <mark.rutland@arm.com> wrote: > > On Mon, Aug 19, 2024 at 06:22:41PM +0100, Andre Przywara wrote: > > > On Mon, 12 Aug 2024 11:15:51 +0100 > > > Mark Rutland <mark.rutland@arm.com> wrote: > > > > @@ -111,23 +108,28 @@ ASM_FUNC(jump_kernel) > > > > bl find_logical_id > > > > bl setup_stack > > > > > > > > - ldr lr, [r5], #4 > > > > - ldm r5, {r0 - r2} > > > > - > > > > - ldr r4, =flag_no_el3 > > > > - ldr r4, [r4] > > > > - cmp r4, #1 > > > > - bxeq lr @ no EL3 > > > > + mov r0, r5 > > > > + mov r1, r6 > > > > + mov r2, r7 > > > > + ldr r3, =SPSR_KERNEL > > > > > > > > - ldr r4, =SPSR_KERNEL > > > > /* Return in thumb2 mode when bit 0 of address is 1 */ > > > > - tst lr, #1 > > > > - orrne r4, #PSR_T > > > > + tst r4, #1 > > > > + orrne r3, #PSR_T > > > > + > > > > + mrs r5, cpsr > > > > + and r5, #PSR_MODE_MASK > > > > + cmp r5, #PSR_MON > > > > + beq eret_at_mon > > > > + cmp r5, #PSR_HYP > > > > + beq eret_at_hyp > > > > + b . > > > > > > > > - msr spsr_cxf, r4 > > > > +eret_at_mon: > > > > + mov lr, r4 > > > > + msr spsr_cxf, r3 > > > > movs pc, lr > > > Reading "B9.1 General restrictions on system instructions" in the ARMv7 ARM > > > I don't immediately see why an eret wouldn't be possible here. > > > > > > If there is a restriction I missed, I guess either a comment here or in > > > the commit message would be helpful. > > > > We can use ERET here; IIRC that was added in the ARMv7 virtualization > > extensions, but the boot-wrapper requires that and really it's ARMv8+ > > Is that so? I mean in all practicality we will indeed use the bootwrapper > on ARMv8 only these days, but I don't think we need to artificially limit > this. Also I consider the boot-wrapper one of the more reliable sources > for ARMv7 boot code, so not sure we should drop this aspect. > There is one ARMv7 compile time check, to avoid "sevl", so we have some > support, at least. What I was trying to say here was "the minimum bound is ARMv7 + virtualization extensions", which is already required by the ".arch_extension virt" directive that's been in this file since it was introduced. Practically speaking, I don't think that we should care about ARMv7 here, but if that happens to work, great! > > anyway. I had opted to stick with "movs pc, lr" because it was a > > (trivially) smaller change, and kept the cases distinct, but I'm happy > > to use ERET. > > > > However, beware that in AArch32 ERET is a bit odd: in Hyp mode takes the > > return address from ELR_HYP, while in all other modes it takes it from > > the LR (as only hyp has an ELR). > > Yeah, I saw this yesterday, and am even more grateful for the ARMv8 > exception model now ;-) > > So I am fine with "movs pc, lr", if that's the more canonical way on > 32-bit/ARMv7. On the other hand your revised sequence below looks > intriguingly simple ... > > > > > > > - > > > > - .section .data > > > > - .align 2 > > > > -flag_no_el3: > > > > - .long 0 > > > > +eret_at_hyp: > > > > + msr elr_hyp, r4 > > > > + msr spsr_cxf, r3 > > > > > > Shouldn't that be spsr_hyp? > > > > It can be, but doesn't need to be. This is the SPSR_<fields> encoding, > > So I didn't know about this until yesterday, and it's not easy to find, > since it seems not to be mentioned as such in the ARM ARM (at least not > "cxf"). binutils seems to disassemble this to SPSR_fxc, but I guess we > should indeed move to SPSR_fsxc (if we keep this at all). > > > which writes to the SPSR for owned by the active mode, though it skips > > bits<23:16>, which we probably should initialise. > > > > If I change that all to: > > > > | eret_at_mon: > > | mov lr, r4 > > | msr spsr_mon, r3 > > | eret > > | eret_at_hyp: > > | msr elr_hyp, r4 > > | msr spsr_hyp, r3 > > | > > > > ... do you think that's clear enough, or do you think we need a comment > > about the "LR" vs "ELR_HYP" distinction? > > Oh, that certainly looks the clearest, but indeed a comment on LR vs. ELR > situation looks indicated. Considering the earlier comments I'm going to make this: | eret_at_mon: | mov lr, r4 | msr spsr_mon | movs pc, lr | eret_at_hyp: | msr elr_hyp, r4 | msr spsr_hyp, r3 | eret Using 'spsr_mon' and 'spsr_hyp' means we initialize *all* of the SPSR bits, so that's a bug fix in addition to being clearer. Using 'movs pc, lr' for the 'eret_at_mon' case is the standard way to do exception returns in AArch32 generally, and then that clearly doesnt't depend on the virtualization extensions, so if we ever want to handle a CPU without hyp in future all we'll need to do is mess with the SPSR value. I'm not going to bother with a comment given that's standard AArch32 behaviour. Mark.
On Tue, 20 Aug 2024 14:36:37 +0100 Mark Rutland <mark.rutland@arm.com> wrote: Hi Mark, > On Tue, Aug 20, 2024 at 01:59:44PM +0100, Andre Przywara wrote: > > On Tue, 20 Aug 2024 12:43:18 +0100 > > Mark Rutland <mark.rutland@arm.com> wrote: > > > On Mon, Aug 19, 2024 at 06:22:41PM +0100, Andre Przywara wrote: > > > > On Mon, 12 Aug 2024 11:15:51 +0100 > > > > Mark Rutland <mark.rutland@arm.com> wrote: > > > > > > @@ -111,23 +108,28 @@ ASM_FUNC(jump_kernel) > > > > > bl find_logical_id > > > > > bl setup_stack > > > > > > > > > > - ldr lr, [r5], #4 > > > > > - ldm r5, {r0 - r2} > > > > > - > > > > > - ldr r4, =flag_no_el3 > > > > > - ldr r4, [r4] > > > > > - cmp r4, #1 > > > > > - bxeq lr @ no EL3 > > > > > + mov r0, r5 > > > > > + mov r1, r6 > > > > > + mov r2, r7 > > > > > + ldr r3, =SPSR_KERNEL > > > > > > > > > > - ldr r4, =SPSR_KERNEL > > > > > /* Return in thumb2 mode when bit 0 of address is 1 */ > > > > > - tst lr, #1 > > > > > - orrne r4, #PSR_T > > > > > + tst r4, #1 > > > > > + orrne r3, #PSR_T > > > > > + > > > > > + mrs r5, cpsr > > > > > + and r5, #PSR_MODE_MASK > > > > > + cmp r5, #PSR_MON > > > > > + beq eret_at_mon > > > > > + cmp r5, #PSR_HYP > > > > > + beq eret_at_hyp > > > > > + b . > > > > > > > > > > - msr spsr_cxf, r4 > > > > > +eret_at_mon: > > > > > + mov lr, r4 > > > > > + msr spsr_cxf, r3 > > > > > movs pc, lr > > > > > Reading "B9.1 General restrictions on system instructions" in the ARMv7 ARM > > > > I don't immediately see why an eret wouldn't be possible here. > > > > > > > > If there is a restriction I missed, I guess either a comment here or in > > > > the commit message would be helpful. > > > > > > We can use ERET here; IIRC that was added in the ARMv7 virtualization > > > extensions, but the boot-wrapper requires that and really it's ARMv8+ > > > > Is that so? I mean in all practicality we will indeed use the bootwrapper > > on ARMv8 only these days, but I don't think we need to artificially limit > > this. Also I consider the boot-wrapper one of the more reliable sources > > for ARMv7 boot code, so not sure we should drop this aspect. > > There is one ARMv7 compile time check, to avoid "sevl", so we have some > > support, at least. > > What I was trying to say here was "the minimum bound is ARMv7 + > virtualization extensions", which is already required by the > ".arch_extension virt" directive that's been in this file since it was > introduced. > > Practically speaking, I don't think that we should care about ARMv7 > here, but if that happens to work, great! Ah, no, I meant "armv7ve". Given that we either drop to HYP or stay in HYP, I don't think supporting something before that makes much sense here ;-) > > > anyway. I had opted to stick with "movs pc, lr" because it was a > > > (trivially) smaller change, and kept the cases distinct, but I'm happy > > > to use ERET. > > > > > > However, beware that in AArch32 ERET is a bit odd: in Hyp mode takes the > > > return address from ELR_HYP, while in all other modes it takes it from > > > the LR (as only hyp has an ELR). > > > > Yeah, I saw this yesterday, and am even more grateful for the ARMv8 > > exception model now ;-) > > > > So I am fine with "movs pc, lr", if that's the more canonical way on > > 32-bit/ARMv7. On the other hand your revised sequence below looks > > intriguingly simple ... > > > > > > > > > > - > > > > > - .section .data > > > > > - .align 2 > > > > > -flag_no_el3: > > > > > - .long 0 > > > > > +eret_at_hyp: > > > > > + msr elr_hyp, r4 > > > > > + msr spsr_cxf, r3 > > > > > > > > Shouldn't that be spsr_hyp? > > > > > > It can be, but doesn't need to be. This is the SPSR_<fields> encoding, > > > > So I didn't know about this until yesterday, and it's not easy to find, > > since it seems not to be mentioned as such in the ARM ARM (at least not > > "cxf"). binutils seems to disassemble this to SPSR_fxc, but I guess we > > should indeed move to SPSR_fsxc (if we keep this at all). > > > > > which writes to the SPSR for owned by the active mode, though it skips > > > bits<23:16>, which we probably should initialise. > > > > > > If I change that all to: > > > > > > | eret_at_mon: > > > | mov lr, r4 > > > | msr spsr_mon, r3 > > > | eret > > > | eret_at_hyp: > > > | msr elr_hyp, r4 > > > | msr spsr_hyp, r3 > > > | > > > > > > ... do you think that's clear enough, or do you think we need a comment > > > about the "LR" vs "ELR_HYP" distinction? > > > > Oh, that certainly looks the clearest, but indeed a comment on LR vs. ELR > > situation looks indicated. > > Considering the earlier comments I'm going to make this: > > | eret_at_mon: > | mov lr, r4 > | msr spsr_mon > | movs pc, lr > | eret_at_hyp: > | msr elr_hyp, r4 > | msr spsr_hyp, r3 > | eret > > Using 'spsr_mon' and 'spsr_hyp' means we initialize *all* of the SPSR > bits, so that's a bug fix in addition to being clearer. > > Using 'movs pc, lr' for the 'eret_at_mon' case is the standard way to do > exception returns in AArch32 generally, and then that clearly doesnt't > depend on the virtualization extensions, so if we ever want to handle a > CPU without hyp in future all we'll need to do is mess with the SPSR > value. > > I'm not going to bother with a comment given that's standard AArch32 > behaviour. Many thanks, that looks absolutely fine to me and makes the most sense! Cheers, Andre.
diff --git a/arch/aarch32/boot.S b/arch/aarch32/boot.S index f21f89a..78d19a0 100644 --- a/arch/aarch32/boot.S +++ b/arch/aarch32/boot.S @@ -76,10 +76,6 @@ reset_at_hyp: bl setup_stack - mov r0, #1 - ldr r1, =flag_no_el3 - str r0, [r1] - bl cpu_init_bootwrapper bl cpu_init_arch @@ -96,9 +92,10 @@ err_invalid_id: * r1-r3, sp[0]: kernel arguments */ ASM_FUNC(jump_kernel) - sub sp, #4 @ Ignore fourth argument - push {r0 - r3} - mov r5, sp + mov r4, r0 + mov r5, r1 + mov r6, r2 + mov r7, r3 ldr r0, =HSCTLR_KERNEL mcr p15, 4, r0, c1, c0, 0 @ HSCTLR @@ -111,23 +108,28 @@ ASM_FUNC(jump_kernel) bl find_logical_id bl setup_stack - ldr lr, [r5], #4 - ldm r5, {r0 - r2} - - ldr r4, =flag_no_el3 - ldr r4, [r4] - cmp r4, #1 - bxeq lr @ no EL3 + mov r0, r5 + mov r1, r6 + mov r2, r7 + ldr r3, =SPSR_KERNEL - ldr r4, =SPSR_KERNEL /* Return in thumb2 mode when bit 0 of address is 1 */ - tst lr, #1 - orrne r4, #PSR_T + tst r4, #1 + orrne r3, #PSR_T + + mrs r5, cpsr + and r5, #PSR_MODE_MASK + cmp r5, #PSR_MON + beq eret_at_mon + cmp r5, #PSR_HYP + beq eret_at_hyp + b . - msr spsr_cxf, r4 +eret_at_mon: + mov lr, r4 + msr spsr_cxf, r3 movs pc, lr - - .section .data - .align 2 -flag_no_el3: - .long 0 +eret_at_hyp: + msr elr_hyp, r4 + msr spsr_cxf, r3 + eret