| Message ID | 20190118212037.24412-4-sean.j.christopherson@intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: VMX: Move vCPU-run to proper asm sub-routine | expand |
On Fri, Jan 18, 2019 at 1:22 PM Sean Christopherson <sean.j.christopherson@intel.com> wrote: > > In the vCPU-run asm blob, the guest's RCX is temporarily saved onto the > stack after VM-Exit as the exit flow must first load a register with a > pointer to the vCPU's save area in order to save the guest's registers. > RCX is arbitrarily designated as the scratch register. > > Since the stack usage is to (1)save host, (2)save guest, (3)load host > and (4)load guest, the code can't conform to the stack's natural FIFO > semantics, i.e. it can't simply do PUSH/POP. Regardless of whether it > is done for the host's value or guest's value, at some point the code > needs to access the stack using a non-traditional method, e.g. MOV > instead of POP. vCPU-run opts to create a placeholder on the stack for > guest's RCX (by adjusting RSP) and saves RCX to its place immediately > after VM-Exit (via MOV). > > In other words, the purpose of the first 'PUSH RCX' at the start of > the vCPU-run asm blob is to adjust RSP down, i.e. there's no need to > actually access memory. Use 'SUB $wordsize, RSP' instead of 'PUSH RCX' > to make it more obvious that the intent is simply to create a gap on > the stack for the guest's RCX. > > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com> Reviewed-by: Jim Mattson <jmattson@google.com>
On Fri, Jan 18, 2019 at 01:20:11PM -0800, Sean Christopherson wrote: > In the vCPU-run asm blob, the guest's RCX is temporarily saved onto the > stack after VM-Exit as the exit flow must first load a register with a > pointer to the vCPU's save area in order to save the guest's registers. > RCX is arbitrarily designated as the scratch register. > > Since the stack usage is to (1)save host, (2)save guest, (3)load host > and (4)load guest, the code can't conform to the stack's natural FIFO > semantics, i.e. it can't simply do PUSH/POP. Regardless of whether it > is done for the host's value or guest's value, at some point the code > needs to access the stack using a non-traditional method, e.g. MOV > instead of POP. vCPU-run opts to create a placeholder on the stack for > guest's RCX (by adjusting RSP) and saves RCX to its place immediately > after VM-Exit (via MOV). > > In other words, the purpose of the first 'PUSH RCX' at the start of > the vCPU-run asm blob is to adjust RSP down, i.e. there's no need to > actually access memory. Use 'SUB $wordsize, RSP' instead of 'PUSH RCX' > to make it more obvious that the intent is simply to create a gap on > the stack for the guest's RCX. > > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com> Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> Thank you! > --- > arch/x86/kvm/vmx/vmx.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c > index 23e5fa58751a..adf59fd23a6c 100644 > --- a/arch/x86/kvm/vmx/vmx.c > +++ b/arch/x86/kvm/vmx/vmx.c > @@ -6377,7 +6377,7 @@ static void __vmx_vcpu_run(struct kvm_vcpu *vcpu, struct vcpu_vmx *vmx) > asm( > /* Store host registers */ > "push %%" _ASM_DX "; push %%" _ASM_BP ";" > - "push %%" _ASM_CX " \n\t" /* placeholder for guest rcx */ > + "sub $%c[wordsize], %%" _ASM_SP "\n\t" /* placeholder for guest RCX */ > "push %%" _ASM_CX " \n\t" > "sub $%c[wordsize], %%" _ASM_SP "\n\t" /* temporarily adjust RSP for CALL */ > "cmp %%" _ASM_SP ", %c[host_rsp](%%" _ASM_CX ") \n\t" > -- > 2.20.1 >
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c index 23e5fa58751a..adf59fd23a6c 100644 --- a/arch/x86/kvm/vmx/vmx.c +++ b/arch/x86/kvm/vmx/vmx.c @@ -6377,7 +6377,7 @@ static void __vmx_vcpu_run(struct kvm_vcpu *vcpu, struct vcpu_vmx *vmx) asm( /* Store host registers */ "push %%" _ASM_DX "; push %%" _ASM_BP ";" - "push %%" _ASM_CX " \n\t" /* placeholder for guest rcx */ + "sub $%c[wordsize], %%" _ASM_SP "\n\t" /* placeholder for guest RCX */ "push %%" _ASM_CX " \n\t" "sub $%c[wordsize], %%" _ASM_SP "\n\t" /* temporarily adjust RSP for CALL */ "cmp %%" _ASM_SP ", %c[host_rsp](%%" _ASM_CX ") \n\t"
In the vCPU-run asm blob, the guest's RCX is temporarily saved onto the stack after VM-Exit as the exit flow must first load a register with a pointer to the vCPU's save area in order to save the guest's registers. RCX is arbitrarily designated as the scratch register. Since the stack usage is to (1)save host, (2)save guest, (3)load host and (4)load guest, the code can't conform to the stack's natural FIFO semantics, i.e. it can't simply do PUSH/POP. Regardless of whether it is done for the host's value or guest's value, at some point the code needs to access the stack using a non-traditional method, e.g. MOV instead of POP. vCPU-run opts to create a placeholder on the stack for guest's RCX (by adjusting RSP) and saves RCX to its place immediately after VM-Exit (via MOV). In other words, the purpose of the first 'PUSH RCX' at the start of the vCPU-run asm blob is to adjust RSP down, i.e. there's no need to actually access memory. Use 'SUB $wordsize, RSP' instead of 'PUSH RCX' to make it more obvious that the intent is simply to create a gap on the stack for the guest's RCX. Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com> --- arch/x86/kvm/vmx/vmx.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)