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Message-ID: <Y2r+/UYNeZ7ljYXC@google.com>
Date: Wed, 9 Nov 2022 01:14:37 +0000
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
nathan@...nel.org, thomas.lendacky@....com,
andrew.cooper3@...rix.com, peterz@...radead.org,
jmattson@...gle.com, stable@...r.kernel.org
Subject: Re: [PATCH v2 7/8] KVM: SVM: move MSR_IA32_SPEC_CTRL save/restore to
assembly
On Tue, Nov 08, 2022, Paolo Bonzini wrote:
> diff --git a/arch/x86/kvm/svm/vmenter.S b/arch/x86/kvm/svm/vmenter.S
> index 0a4272faf80f..a02eef724379 100644
> --- a/arch/x86/kvm/svm/vmenter.S
> +++ b/arch/x86/kvm/svm/vmenter.S
> @@ -32,10 +32,70 @@
>
> .section .noinstr.text, "ax"
>
> +.macro RESTORE_GUEST_SPEC_CTRL
> + /* No need to do anything if SPEC_CTRL is unset or V_SPEC_CTRL is set */
> + ALTERNATIVE_2 "", \
> + "jmp 800f", X86_FEATURE_MSR_SPEC_CTRL, \
> + "", X86_FEATURE_V_SPEC_CTRL
> +801:
> +.endm
> +
> +.macro RESTORE_HOST_SPEC_CTRL
> + /* No need to do anything if SPEC_CTRL is unset or V_SPEC_CTRL is set */
> + ALTERNATIVE_2 "", \
> + "jmp 900f", X86_FEATURE_MSR_SPEC_CTRL, \
> + "", X86_FEATURE_V_SPEC_CTRL
> +901:
> +.endm
> +
> +.macro RESTORE_SPEC_CTRL_BODY
Can we split these into separate macros? It's a bit more typing, but it's not
immediately obvious that these are two independent chunks (I was expecting a JMP
from the 800 section into the 900 section).
E.g. RESTORE_GUEST_SPEC_CTRL_BODY and RESTORE_HOST_SPEC_CTRL_BODY
> +800:
Ugh, the multiple users makes it somewhat ugly, but rather than arbitrary numbers,
what about using named labels to make it easier to understand the branches?
E.g.
---
arch/x86/kvm/svm/vmenter.S | 43 +++++++++++++++++++++-----------------
1 file changed, 24 insertions(+), 19 deletions(-)
diff --git a/arch/x86/kvm/svm/vmenter.S b/arch/x86/kvm/svm/vmenter.S
index a02eef724379..23fd7353f0d0 100644
--- a/arch/x86/kvm/svm/vmenter.S
+++ b/arch/x86/kvm/svm/vmenter.S
@@ -32,24 +32,24 @@
.section .noinstr.text, "ax"
-.macro RESTORE_GUEST_SPEC_CTRL
+.macro RESTORE_GUEST_SPEC_CTRL name
/* No need to do anything if SPEC_CTRL is unset or V_SPEC_CTRL is set */
ALTERNATIVE_2 "", \
- "jmp 800f", X86_FEATURE_MSR_SPEC_CTRL, \
+ "jmp .Lrestore_guest_spec_ctrl\name", X86_FEATURE_MSR_SPEC_CTRL, \
"", X86_FEATURE_V_SPEC_CTRL
-801:
+.Lpost_restore_guest_spec_ctrl\name:
.endm
-.macro RESTORE_HOST_SPEC_CTRL
+.macro RESTORE_HOST_SPEC_CTRL name
/* No need to do anything if SPEC_CTRL is unset or V_SPEC_CTRL is set */
ALTERNATIVE_2 "", \
- "jmp 900f", X86_FEATURE_MSR_SPEC_CTRL, \
+ "jmp .Lrestore_host_spec_ctrl\name", X86_FEATURE_MSR_SPEC_CTRL, \
"", X86_FEATURE_V_SPEC_CTRL
-901:
+.Lpost_restore_host_spec_ctrl\name:
.endm
-.macro RESTORE_SPEC_CTRL_BODY
-800:
+.macro RESTORE_GUEST_SPEC_CTRL_BODY name
+.Lrestore_guest_spec_ctrl\name:
/*
* SPEC_CTRL handling: if the guest's SPEC_CTRL value differs from the
* host's, write the MSR. This is kept out-of-line so that the common
@@ -61,13 +61,16 @@
*/
movl SVM_spec_ctrl(%_ASM_DI), %eax
cmp PER_CPU_VAR(x86_spec_ctrl_current), %eax
- je 801b
+ je .Lpost_restore_guest_spec_ctrl\name
+
mov $MSR_IA32_SPEC_CTRL, %ecx
xor %edx, %edx
wrmsr
- jmp 801b
+ jmp .Lpost_restore_guest_spec_ctrl\name
+.endm
-900:
+.macro RESTORE_HOST_SPEC_CTRL_BODY name
+.Lrestore_host_spec_ctrl\name:
/* Same for after vmexit. */
mov $MSR_IA32_SPEC_CTRL, %ecx
@@ -76,18 +79,18 @@
* if it was not intercepted during guest execution.
*/
cmpb $0, (%_ASM_SP)
- jnz 998f
+ jnz .Lskip_save_guest_spec_ctrl\name
rdmsr
movl %eax, SVM_spec_ctrl(%_ASM_DI)
-998:
+.Lskip_save_guest_spec_ctrl\name:
/* Now restore the host value of the MSR if different from the guest's. */
movl PER_CPU_VAR(x86_spec_ctrl_current), %eax
cmp SVM_spec_ctrl(%_ASM_DI), %eax
- je 901b
+ je .Lpost_restore_host_spec_ctrl\name
xor %edx, %edx
wrmsr
- jmp 901b
+ jmp .Lpost_restore_host_spec_ctrl\name
.endm
@@ -259,7 +262,8 @@ SYM_FUNC_START(__svm_vcpu_run)
pop %_ASM_BP
RET
- RESTORE_SPEC_CTRL_BODY
+ RESTORE_GUEST_SPEC_CTRL_BODY
+ RESTORE_HOST_SPEC_CTRL_BODY
10: cmpb $0, kvm_rebooting
jne 2b
@@ -310,7 +314,7 @@ SYM_FUNC_START(__svm_sev_es_vcpu_run)
mov %_ASM_ARG1, %_ASM_DI
.endif
- RESTORE_GUEST_SPEC_CTRL
+ RESTORE_GUEST_SPEC_CTRL sev_es
/* Get svm->current_vmcb->pa into RAX. */
mov SVM_current_vmcb(%_ASM_DI), %_ASM_AX
@@ -331,7 +335,7 @@ SYM_FUNC_START(__svm_sev_es_vcpu_run)
FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, X86_FEATURE_RETPOLINE
#endif
- RESTORE_HOST_SPEC_CTRL
+ RESTORE_HOST_SPEC_CTRL sev_es
/*
* Mitigate RETBleed for AMD/Hygon Zen uarch. RET should be
@@ -359,7 +363,8 @@ SYM_FUNC_START(__svm_sev_es_vcpu_run)
pop %_ASM_BP
RET
- RESTORE_SPEC_CTRL_BODY
+ RESTORE_GUEST_SPEC_CTRL_BODY sev_es
+ RESTORE_HOST_SPEC_CTRL_BODY sev_es
3: cmpb $0, kvm_rebooting
jne 2b
base-commit: 0b242ada175d97a556866c48c80310860f634579
--
> @@ -61,6 +126,8 @@ SYM_FUNC_START(__svm_vcpu_run)
> mov %_ASM_ARG1, %_ASM_DI
> .endif
>
Can you add a comment to all of these to call out that RAX, RCX, and RDX might
get clobbered? It's easy to overlook that detail, and it matters on 32-bit where
the arguments use RCX and RDX. And because the clobber is conditional, it wouldn't
be that hard for a bug to escape initial testing.
/* This might clobber RAX, RCX, and RDX! */
> + RESTORE_GUEST_SPEC_CTRL
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