| Message ID | 20241011085244.432368-6-frediano.ziglio@cloud.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | Reuse 32 bit C code more safely | expand |
On Fri, Oct 11, 2024 at 09:52:44AM +0100, Frediano Ziglio wrote: > Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com> > --- > xen/arch/x86/boot/reloc.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/xen/arch/x86/boot/reloc.c b/xen/arch/x86/boot/reloc.c > index e50e161b27..e725cfb6eb 100644 > --- a/xen/arch/x86/boot/reloc.c > +++ b/xen/arch/x86/boot/reloc.c > @@ -65,7 +65,7 @@ typedef struct memctx { > /* > * Simple bump allocator. > * > - * It starts from the base of the trampoline and allocates downwards. > + * It starts on top of space reserved for the trampoline and allocates downwards. nit: Not sure this is much clearer. The trampoline is not a stack (and even if it was, I personally find "top" and "bottom" quite ambiguous when it grows backwards), so calling top to its lowest address seems more confusing than not. If anything clarification ought to go in the which direction it takes. Leaving "base" instead of "top" and replacing "downwards" by "backwards" to make it crystal clear that it's a pointer that starts where the trampoline starts, but moves in the opposite direction. My .02 anyway. > */ > uint32_t ptr; > } memctx; > -- > 2.34.1 > > Cheers, Alejandro
On Fri, Oct 11, 2024 at 1:56 PM Alejandro Vallejo <alejandro.vallejo@cloud.com> wrote: > > On Fri, Oct 11, 2024 at 09:52:44AM +0100, Frediano Ziglio wrote: > > Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com> > > --- > > xen/arch/x86/boot/reloc.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/xen/arch/x86/boot/reloc.c b/xen/arch/x86/boot/reloc.c > > index e50e161b27..e725cfb6eb 100644 > > --- a/xen/arch/x86/boot/reloc.c > > +++ b/xen/arch/x86/boot/reloc.c > > @@ -65,7 +65,7 @@ typedef struct memctx { > > /* > > * Simple bump allocator. > > * > > - * It starts from the base of the trampoline and allocates downwards. > > + * It starts on top of space reserved for the trampoline and allocates downwards. > > nit: Not sure this is much clearer. The trampoline is not a stack (and even if > it was, I personally find "top" and "bottom" quite ambiguous when it grows > backwards), so calling top to its lowest address seems more confusing than not. > > If anything clarification ought to go in the which direction it takes. Leaving > "base" instead of "top" and replacing "downwards" by "backwards" to make it > crystal clear that it's a pointer that starts where the trampoline starts, but > moves in the opposite direction. > Base looks confusing to me, but surely that comment could be confusing. For the trampoline 64 KB are reserved. Last 4 KB are used as a normal stack (push/pop/call/whatever), first part gets a copy of the trampoline code/data (about 6 Kb) the rest (so 64 - 4 - ~6 = ~54 kb) is used for the copy of MBI information. That "rest" is what we are talking about here. > My .02 anyway. > > > */ > > uint32_t ptr; > > } memctx; > > -- > > 2.34.1 > > > > > > Cheers, > Alejandro Frediano
On Fri, Oct 11, 2024 at 02:08:37PM +0100, Frediano Ziglio wrote: > On Fri, Oct 11, 2024 at 1:56 PM Alejandro Vallejo > <alejandro.vallejo@cloud.com> wrote: > > > > On Fri, Oct 11, 2024 at 09:52:44AM +0100, Frediano Ziglio wrote: > > > Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com> > > > --- > > > xen/arch/x86/boot/reloc.c | 2 +- > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > diff --git a/xen/arch/x86/boot/reloc.c b/xen/arch/x86/boot/reloc.c > > > index e50e161b27..e725cfb6eb 100644 > > > --- a/xen/arch/x86/boot/reloc.c > > > +++ b/xen/arch/x86/boot/reloc.c > > > @@ -65,7 +65,7 @@ typedef struct memctx { > > > /* > > > * Simple bump allocator. > > > * > > > - * It starts from the base of the trampoline and allocates downwards. > > > + * It starts on top of space reserved for the trampoline and allocates downwards. > > > > nit: Not sure this is much clearer. The trampoline is not a stack (and even if > > it was, I personally find "top" and "bottom" quite ambiguous when it grows > > backwards), so calling top to its lowest address seems more confusing than not. > > > > If anything clarification ought to go in the which direction it takes. Leaving > > "base" instead of "top" and replacing "downwards" by "backwards" to make it > > crystal clear that it's a pointer that starts where the trampoline starts, but > > moves in the opposite direction. > > > > Base looks confusing to me, but surely that comment could be confusing. > For the trampoline 64 KB are reserved. Last 4 KB are used as a normal > stack (push/pop/call/whatever), first part gets a copy of the > trampoline code/data (about 6 Kb) the rest (so 64 - 4 - ~6 = ~54 kb) > is used for the copy of MBI information. That "rest" is what we are > talking about here. Last? From what I looked at it seems to be the first 12K. #define TRAMPOLINE_STACK_SPACE PAGE_SIZE #define TRAMPOLINE_SPACE (KB(64) - TRAMPOLINE_STACK_SPACE) To put it another way, with left=lo-addr and right=hi-addr. The code seems to do this... |<--------------64K-------------->| |<-----12K--->| | +-------------+-----+-------------+ | stack-space | mbi | trampoline | +-------------+-----+-------------+ ^ ^ | | | +-- copied Multiboot info + modules +----- initial memctx.ptr ... with the stack growing backwards to avoid overflowing onto mbi. Or am I missing something? Cheers, Alejandro
On 11/10/2024 2:28 pm, Alejandro Vallejo wrote: > On Fri, Oct 11, 2024 at 02:08:37PM +0100, Frediano Ziglio wrote: >> On Fri, Oct 11, 2024 at 1:56 PM Alejandro Vallejo >> <alejandro.vallejo@cloud.com> wrote: >>> On Fri, Oct 11, 2024 at 09:52:44AM +0100, Frediano Ziglio wrote: >>>> Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com> >>>> --- >>>> xen/arch/x86/boot/reloc.c | 2 +- >>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>> >>>> diff --git a/xen/arch/x86/boot/reloc.c b/xen/arch/x86/boot/reloc.c >>>> index e50e161b27..e725cfb6eb 100644 >>>> --- a/xen/arch/x86/boot/reloc.c >>>> +++ b/xen/arch/x86/boot/reloc.c >>>> @@ -65,7 +65,7 @@ typedef struct memctx { >>>> /* >>>> * Simple bump allocator. >>>> * >>>> - * It starts from the base of the trampoline and allocates downwards. >>>> + * It starts on top of space reserved for the trampoline and allocates downwards. >>> nit: Not sure this is much clearer. The trampoline is not a stack (and even if >>> it was, I personally find "top" and "bottom" quite ambiguous when it grows >>> backwards), so calling top to its lowest address seems more confusing than not. >>> >>> If anything clarification ought to go in the which direction it takes. Leaving >>> "base" instead of "top" and replacing "downwards" by "backwards" to make it >>> crystal clear that it's a pointer that starts where the trampoline starts, but >>> moves in the opposite direction. >>> >> Base looks confusing to me, but surely that comment could be confusing. >> For the trampoline 64 KB are reserved. Last 4 KB are used as a normal >> stack (push/pop/call/whatever), first part gets a copy of the >> trampoline code/data (about 6 Kb) the rest (so 64 - 4 - ~6 = ~54 kb) >> is used for the copy of MBI information. That "rest" is what we are >> talking about here. > Last? From what I looked at it seems to be the first 12K. > > #define TRAMPOLINE_STACK_SPACE PAGE_SIZE > #define TRAMPOLINE_SPACE (KB(64) - TRAMPOLINE_STACK_SPACE) > > To put it another way, with left=lo-addr and right=hi-addr. The code seems to > do this... > > |<--------------64K-------------->| > |<-----12K--->| | > +-------------+-----+-------------+ > | stack-space | mbi | trampoline | > +-------------+-----+-------------+ > ^ ^ > | | > | +-- copied Multiboot info + modules > +----- initial memctx.ptr > > ... with the stack growing backwards to avoid overflowing onto mbi. > > Or am I missing something? So I was hoping for some kind of diagram like this, to live in arch/x86/include/asm/trampoline.h with the other notes about the trampoline. But, is that diagram accurate? Looking at
On Fri, Oct 11, 2024 at 2:38 PM Andrew Cooper <andrew.cooper3@citrix.com> wrote: > > On 11/10/2024 2:28 pm, Alejandro Vallejo wrote: > > On Fri, Oct 11, 2024 at 02:08:37PM +0100, Frediano Ziglio wrote: > >> On Fri, Oct 11, 2024 at 1:56 PM Alejandro Vallejo > >> <alejandro.vallejo@cloud.com> wrote: > >>> On Fri, Oct 11, 2024 at 09:52:44AM +0100, Frediano Ziglio wrote: > >>>> Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com> > >>>> --- > >>>> xen/arch/x86/boot/reloc.c | 2 +- > >>>> 1 file changed, 1 insertion(+), 1 deletion(-) > >>>> > >>>> diff --git a/xen/arch/x86/boot/reloc.c b/xen/arch/x86/boot/reloc.c > >>>> index e50e161b27..e725cfb6eb 100644 > >>>> --- a/xen/arch/x86/boot/reloc.c > >>>> +++ b/xen/arch/x86/boot/reloc.c > >>>> @@ -65,7 +65,7 @@ typedef struct memctx { > >>>> /* > >>>> * Simple bump allocator. > >>>> * > >>>> - * It starts from the base of the trampoline and allocates downwards. > >>>> + * It starts on top of space reserved for the trampoline and allocates downwards. > >>> nit: Not sure this is much clearer. The trampoline is not a stack (and even if > >>> it was, I personally find "top" and "bottom" quite ambiguous when it grows > >>> backwards), so calling top to its lowest address seems more confusing than not. > >>> > >>> If anything clarification ought to go in the which direction it takes. Leaving > >>> "base" instead of "top" and replacing "downwards" by "backwards" to make it > >>> crystal clear that it's a pointer that starts where the trampoline starts, but > >>> moves in the opposite direction. > >>> > >> Base looks confusing to me, but surely that comment could be confusing. > >> For the trampoline 64 KB are reserved. Last 4 KB are used as a normal > >> stack (push/pop/call/whatever), first part gets a copy of the > >> trampoline code/data (about 6 Kb) the rest (so 64 - 4 - ~6 = ~54 kb) > >> is used for the copy of MBI information. That "rest" is what we are > >> talking about here. > > Last? From what I looked at it seems to be the first 12K. > > > > #define TRAMPOLINE_STACK_SPACE PAGE_SIZE > > #define TRAMPOLINE_SPACE (KB(64) - TRAMPOLINE_STACK_SPACE) > > > > To put it another way, with left=lo-addr and right=hi-addr. The code seems to > > do this... > > > > |<--------------64K-------------->| > > |<-----12K--->| | > > +-------------+-----+-------------+ > > | stack-space | mbi | trampoline | > > +-------------+-----+-------------+ > > ^ ^ > > | | > > | +-- copied Multiboot info + modules > > +----- initial memctx.ptr > > > > ... with the stack growing backwards to avoid overflowing onto mbi. > > > > Or am I missing something? > > So I was hoping for some kind of diagram like this, to live in > arch/x86/include/asm/trampoline.h with the other notes about the trampoline. > > But, is that diagram accurate? Looking at /* Switch to low-memory stack which lives at the end of trampoline region. */ mov sym_esi(trampoline_phys), %edi lea TRAMPOLINE_SPACE+TRAMPOLINE_STACK_SPACE(%edi),%esp lea trampoline_boot_cpu_entry-trampoline_start(%edi),%eax pushl $BOOT_CS32 push %eax /* Copy bootstrap trampoline to low memory, below 1MB. */ lea sym_esi(trampoline_start), %esi mov $((trampoline_end - trampoline_start) / 4),%ecx rep movsl So, from low to high - trampoline code/data (%edi at beginning of copy is trampoline_phys, %esi is trampoline_start) - space (used for MBI copy) - stack (%esp is set to trampoline_phys + TRAMPOLINE_SPACE + TRAMPOLINE_STACK_SPACE) Frediano
On 11/10/2024 2:38 pm, Andrew Cooper wrote: > On 11/10/2024 2:28 pm, Alejandro Vallejo wrote: >> On Fri, Oct 11, 2024 at 02:08:37PM +0100, Frediano Ziglio wrote: >>> On Fri, Oct 11, 2024 at 1:56 PM Alejandro Vallejo >>> <alejandro.vallejo@cloud.com> wrote: >>>> On Fri, Oct 11, 2024 at 09:52:44AM +0100, Frediano Ziglio wrote: >>>>> Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com> >>>>> --- >>>>> xen/arch/x86/boot/reloc.c | 2 +- >>>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>>> >>>>> diff --git a/xen/arch/x86/boot/reloc.c b/xen/arch/x86/boot/reloc.c >>>>> index e50e161b27..e725cfb6eb 100644 >>>>> --- a/xen/arch/x86/boot/reloc.c >>>>> +++ b/xen/arch/x86/boot/reloc.c >>>>> @@ -65,7 +65,7 @@ typedef struct memctx { >>>>> /* >>>>> * Simple bump allocator. >>>>> * >>>>> - * It starts from the base of the trampoline and allocates downwards. >>>>> + * It starts on top of space reserved for the trampoline and allocates downwards. >>>> nit: Not sure this is much clearer. The trampoline is not a stack (and even if >>>> it was, I personally find "top" and "bottom" quite ambiguous when it grows >>>> backwards), so calling top to its lowest address seems more confusing than not. >>>> >>>> If anything clarification ought to go in the which direction it takes. Leaving >>>> "base" instead of "top" and replacing "downwards" by "backwards" to make it >>>> crystal clear that it's a pointer that starts where the trampoline starts, but >>>> moves in the opposite direction. >>>> >>> Base looks confusing to me, but surely that comment could be confusing. >>> For the trampoline 64 KB are reserved. Last 4 KB are used as a normal >>> stack (push/pop/call/whatever), first part gets a copy of the >>> trampoline code/data (about 6 Kb) the rest (so 64 - 4 - ~6 = ~54 kb) >>> is used for the copy of MBI information. That "rest" is what we are >>> talking about here. >> Last? From what I looked at it seems to be the first 12K. >> >> #define TRAMPOLINE_STACK_SPACE PAGE_SIZE >> #define TRAMPOLINE_SPACE (KB(64) - TRAMPOLINE_STACK_SPACE) >> >> To put it another way, with left=lo-addr and right=hi-addr. The code seems to >> do this... >> >> |<--------------64K-------------->| >> |<-----12K--->| | >> +-------------+-----+-------------+ >> | stack-space | mbi | trampoline | >> +-------------+-----+-------------+ >> ^ ^ >> | | >> | +-- copied Multiboot info + modules >> +----- initial memctx.ptr >> >> ... with the stack growing backwards to avoid overflowing onto mbi. >> >> Or am I missing something? > So I was hoping for some kind of diagram like this, to live in > arch/x86/include/asm/trampoline.h with the other notes about the trampoline. > > But, is that diagram accurate? Looking at Sorry, sent too early. GDB says that trampoline_end-trampoline_start is 6512, so one and a half pages. TRAMPOLINE_STACK_SPACE is 4k, and subtracted from 64k to make TRAMPOLINE_SPACE So this is why code uses TRAMPOLINE_SPACE+TRAMPOLINE_STACK_SPACE (== 64k) for it's size calculation. Within that, we've got MBI_SPACE_MIN (8k) used in the linker assertion, for SPACE (60k) - (end - start)(~7k). What we're really saying is that the total size is 64k, with the top 4k being stack, the bottom 7k being .text(ish), and the middle 53k being the MBI relocation area. And memctx is a backwards bump allocator within the middle 53k. Maybe we should defer this until after the Hyperlaunch BootInfo series has come in. Later parts have a major change on how we handle the MBI block, and I can see some substantial pruning opportunities. ~Andrew
On Fri Oct 11, 2024 at 2:58 PM BST, Frediano Ziglio wrote: > On Fri, Oct 11, 2024 at 2:38 PM Andrew Cooper <andrew.cooper3@citrix.com> wrote: > > > > On 11/10/2024 2:28 pm, Alejandro Vallejo wrote: > > > On Fri, Oct 11, 2024 at 02:08:37PM +0100, Frediano Ziglio wrote: > > >> On Fri, Oct 11, 2024 at 1:56 PM Alejandro Vallejo > > >> <alejandro.vallejo@cloud.com> wrote: > > >>> On Fri, Oct 11, 2024 at 09:52:44AM +0100, Frediano Ziglio wrote: > > >>>> Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com> > > >>>> --- > > >>>> xen/arch/x86/boot/reloc.c | 2 +- > > >>>> 1 file changed, 1 insertion(+), 1 deletion(-) > > >>>> > > >>>> diff --git a/xen/arch/x86/boot/reloc.c b/xen/arch/x86/boot/reloc.c > > >>>> index e50e161b27..e725cfb6eb 100644 > > >>>> --- a/xen/arch/x86/boot/reloc.c > > >>>> +++ b/xen/arch/x86/boot/reloc.c > > >>>> @@ -65,7 +65,7 @@ typedef struct memctx { > > >>>> /* > > >>>> * Simple bump allocator. > > >>>> * > > >>>> - * It starts from the base of the trampoline and allocates downwards. > > >>>> + * It starts on top of space reserved for the trampoline and allocates downwards. > > >>> nit: Not sure this is much clearer. The trampoline is not a stack (and even if > > >>> it was, I personally find "top" and "bottom" quite ambiguous when it grows > > >>> backwards), so calling top to its lowest address seems more confusing than not. > > >>> > > >>> If anything clarification ought to go in the which direction it takes. Leaving > > >>> "base" instead of "top" and replacing "downwards" by "backwards" to make it > > >>> crystal clear that it's a pointer that starts where the trampoline starts, but > > >>> moves in the opposite direction. > > >>> > > >> Base looks confusing to me, but surely that comment could be confusing. > > >> For the trampoline 64 KB are reserved. Last 4 KB are used as a normal > > >> stack (push/pop/call/whatever), first part gets a copy of the > > >> trampoline code/data (about 6 Kb) the rest (so 64 - 4 - ~6 = ~54 kb) > > >> is used for the copy of MBI information. That "rest" is what we are > > >> talking about here. > > > Last? From what I looked at it seems to be the first 12K. > > > > > > #define TRAMPOLINE_STACK_SPACE PAGE_SIZE > > > #define TRAMPOLINE_SPACE (KB(64) - TRAMPOLINE_STACK_SPACE) > > > > > > To put it another way, with left=lo-addr and right=hi-addr. The code seems to > > > do this... > > > > > > |<--------------64K-------------->| > > > |<-----12K--->| | s/12K/4K/ My brain merged the 12bits in the wrong place. Too much bit twiddling. > > > +-------------+-----+-------------+ > > > | stack-space | mbi | trampoline | > > > +-------------+-----+-------------+ > > > ^ ^ > > > | | > > > | +-- copied Multiboot info + modules > > > +----- initial memctx.ptr > > > > > > ... with the stack growing backwards to avoid overflowing onto mbi. > > > > > > Or am I missing something? > > > > So I was hoping for some kind of diagram like this, to live in > > arch/x86/include/asm/trampoline.h with the other notes about the trampoline. > > > > But, is that diagram accurate? Looking at > > /* Switch to low-memory stack which lives at the end of > trampoline region. */ > mov sym_esi(trampoline_phys), %edi > lea TRAMPOLINE_SPACE+TRAMPOLINE_STACK_SPACE(%edi),%esp > lea trampoline_boot_cpu_entry-trampoline_start(%edi),%eax > pushl $BOOT_CS32 > push %eax > > /* Copy bootstrap trampoline to low memory, below 1MB. */ > lea sym_esi(trampoline_start), %esi > mov $((trampoline_end - trampoline_start) / 4),%ecx > rep movsl > > So, from low to high > - trampoline code/data (%edi at beginning of copy is trampoline_phys, > %esi is trampoline_start) > - space (used for MBI copy) > - stack (%esp is set to trampoline_phys + TRAMPOLINE_SPACE + > TRAMPOLINE_STACK_SPACE) > > Frediano So it's reversed from what I thought |<--------------64K-------------->| | |<-----4K---->| +-------------+-----+-------------+ | text-(ish) | mbi | stack-space | +-------------+-----+-------------+ ^ ^ | | | +-- initial memctx.ptr +------------------- copied Multiboot info + modules Your version of the comment is a definite improvement over the nonsense that was there before. Sorry for the noise :) Cheers, Alejandro
On 11/10/2024 4:06 pm, Alejandro Vallejo wrote: > On Fri Oct 11, 2024 at 2:58 PM BST, Frediano Ziglio wrote: >> On Fri, Oct 11, 2024 at 2:38 PM Andrew Cooper <andrew.cooper3@citrix.com> wrote: >>> On 11/10/2024 2:28 pm, Alejandro Vallejo wrote: >>>> On Fri, Oct 11, 2024 at 02:08:37PM +0100, Frediano Ziglio wrote: >>>>> On Fri, Oct 11, 2024 at 1:56 PM Alejandro Vallejo >>>>> <alejandro.vallejo@cloud.com> wrote: >>>>>> On Fri, Oct 11, 2024 at 09:52:44AM +0100, Frediano Ziglio wrote: >>>>>>> Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com> >>>>>>> --- >>>>>>> xen/arch/x86/boot/reloc.c | 2 +- >>>>>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>>>>> >>>>>>> diff --git a/xen/arch/x86/boot/reloc.c b/xen/arch/x86/boot/reloc.c >>>>>>> index e50e161b27..e725cfb6eb 100644 >>>>>>> --- a/xen/arch/x86/boot/reloc.c >>>>>>> +++ b/xen/arch/x86/boot/reloc.c >>>>>>> @@ -65,7 +65,7 @@ typedef struct memctx { >>>>>>> /* >>>>>>> * Simple bump allocator. >>>>>>> * >>>>>>> - * It starts from the base of the trampoline and allocates downwards. >>>>>>> + * It starts on top of space reserved for the trampoline and allocates downwards. >>>>>> nit: Not sure this is much clearer. The trampoline is not a stack (and even if >>>>>> it was, I personally find "top" and "bottom" quite ambiguous when it grows >>>>>> backwards), so calling top to its lowest address seems more confusing than not. >>>>>> >>>>>> If anything clarification ought to go in the which direction it takes. Leaving >>>>>> "base" instead of "top" and replacing "downwards" by "backwards" to make it >>>>>> crystal clear that it's a pointer that starts where the trampoline starts, but >>>>>> moves in the opposite direction. >>>>>> >>>>> Base looks confusing to me, but surely that comment could be confusing. >>>>> For the trampoline 64 KB are reserved. Last 4 KB are used as a normal >>>>> stack (push/pop/call/whatever), first part gets a copy of the >>>>> trampoline code/data (about 6 Kb) the rest (so 64 - 4 - ~6 = ~54 kb) >>>>> is used for the copy of MBI information. That "rest" is what we are >>>>> talking about here. >>>> Last? From what I looked at it seems to be the first 12K. >>>> >>>> #define TRAMPOLINE_STACK_SPACE PAGE_SIZE >>>> #define TRAMPOLINE_SPACE (KB(64) - TRAMPOLINE_STACK_SPACE) >>>> >>>> To put it another way, with left=lo-addr and right=hi-addr. The code seems to >>>> do this... >>>> >>>> |<--------------64K-------------->| >>>> |<-----12K--->| | > s/12K/4K/ > > My brain merged the 12bits in the wrong place. Too much bit twiddling. > >>>> +-------------+-----+-------------+ >>>> | stack-space | mbi | trampoline | >>>> +-------------+-----+-------------+ >>>> ^ ^ >>>> | | >>>> | +-- copied Multiboot info + modules >>>> +----- initial memctx.ptr >>>> >>>> ... with the stack growing backwards to avoid overflowing onto mbi. >>>> >>>> Or am I missing something? >>> So I was hoping for some kind of diagram like this, to live in >>> arch/x86/include/asm/trampoline.h with the other notes about the trampoline. >>> >>> But, is that diagram accurate? Looking at >> /* Switch to low-memory stack which lives at the end of >> trampoline region. */ >> mov sym_esi(trampoline_phys), %edi >> lea TRAMPOLINE_SPACE+TRAMPOLINE_STACK_SPACE(%edi),%esp >> lea trampoline_boot_cpu_entry-trampoline_start(%edi),%eax >> pushl $BOOT_CS32 >> push %eax >> >> /* Copy bootstrap trampoline to low memory, below 1MB. */ >> lea sym_esi(trampoline_start), %esi >> mov $((trampoline_end - trampoline_start) / 4),%ecx >> rep movsl >> >> So, from low to high >> - trampoline code/data (%edi at beginning of copy is trampoline_phys, >> %esi is trampoline_start) >> - space (used for MBI copy) >> - stack (%esp is set to trampoline_phys + TRAMPOLINE_SPACE + >> TRAMPOLINE_STACK_SPACE) >> >> Frediano > So it's reversed from what I thought > > |<--------------64K-------------->| > | |<-----4K---->| > +-------------+-----+-------------+ > | text-(ish) | mbi | stack-space | > +-------------+-----+-------------+ > ^ ^ > | | > | +-- initial memctx.ptr > +------------------- copied Multiboot info + modules > > > Your version of the comment is a definite improvement over the nonsense that > was there before. Sorry for the noise :) Today, the pointer that becomes memctx.ptr is phys+SPACE, which does not include the stack. So initial memctx.ptr starts immediately below the stack, and the bump allocator goes backwards (leftwards). ~Andrew
diff --git a/xen/arch/x86/boot/reloc.c b/xen/arch/x86/boot/reloc.c index e50e161b27..e725cfb6eb 100644 --- a/xen/arch/x86/boot/reloc.c +++ b/xen/arch/x86/boot/reloc.c @@ -65,7 +65,7 @@ typedef struct memctx { /* * Simple bump allocator. * - * It starts from the base of the trampoline and allocates downwards. + * It starts on top of space reserved for the trampoline and allocates downwards. */ uint32_t ptr; } memctx;
Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com> --- xen/arch/x86/boot/reloc.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)