Re: [PATCH][SVM] Only save/restore MSRs when needed

Anthony Liguori
2007-04-28T05:14:41Z

Anthony Liguori wrote:
> I've tested this with 32-bit and 64-bit guests on a 64-bit host and
> with 32-bit guests on a 32-bit host.
>
> I *think* it's doing the right thing wrt to DEBUGCTL but an extra set
> of eyes would be helpful.

I think the follow patch preserves the current behavior but I'm starting
to believe that the current behavior is wrong.

The current code assumes that the guest DEBUGCTL gets loaded/saved
during vmrun/vmexit. By my reading of the spec (I finally printed out
the new one :-)), DEBUGCTL only gets saved if LBR virtualization is
enabled. LBR virtualization also triggers the save/restore of the other
4 debug registers. The spec seems to suggest that the host version is
saved before the guest is loaded too.

I think the right behavior is to do nothing if LBR is available and
enabled; hw will do it all for us. If LBR isn't available, I think we
should be saving/restoring not just DEBUGCTL but the other 4. We can
further optimize that path though by doing the same sort of tricks I
have below.

If that all seems sane, I'll submit another patch to fix that up.

Regards,

Anthony Liguori

> Depending on my machine's mood, we're at somewhere between 3200 and
> 2700 cycles per VMEXIT.
>
> Regards,
>
> Anthony Liguori
> ------------------------------------------------------------------------
>
> From: Anthony Liguori <aliguori-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>
> Subject: [PATCH][SVM] Only save/restore MSRs when needed
>
> We only have to save/restore MSR_GS_BASE on every VMEXIT. The rest can be
> saved/restored when we leave the VCPU. As a special case, MSR_IA32_DEBUGCTL
> only needs to be saved/restored on every exit if debugging is enabled in
> either the host or the guest.
>
> Signed-off-by: Anthony Liguori <aliguori-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>
>
> Index: kernel/drivers/kvm/svm.c
> ===================================================================
> --- kernel.orig/drivers/kvm/svm.c 2007-04-27 17:52:37.200780944 -0500
> +++ kernel/drivers/kvm/svm.c 2007-04-27 18:14:02.098446808 -0500
> @@ -611,7 +611,7 @@
>
> static void svm_vcpu_load(struct kvm_vcpu *vcpu)
> {
> - int cpu;
> + int cpu, i;
>
> cpu = get_cpu();
> if (unlikely(cpu != vcpu->cpu)) {
> @@ -626,10 +626,24 @@
> vcpu->svm->vmcb->control.tsc_offset += delta;
> vcpu->cpu = cpu;
> }
> +
> + for (i = 0; i < NR_HOST_SAVE_USER_MSRS; i++)
> + rdmsrl(host_save_user_msrs[i], vcpu->svm->host_user_msrs[i]);
> +
> + rdmsrl(MSR_IA32_DEBUGCTLMSR, vcpu->svm->host_ia32_debugctl);
> }
>
> static void svm_vcpu_put(struct kvm_vcpu *vcpu)
> {
> + int i;
> +
> + for (i = 0; i < NR_HOST_SAVE_USER_MSRS; i++)
> + wrmsrl(host_save_user_msrs[i], vcpu->svm->host_user_msrs[i]);
> +
> + wrmsrl(MSR_IA32_DEBUGCTLMSR, vcpu->svm->host_ia32_debugctl);
> + /* FIXME: LASTBRANCHFROMIP, LASTBRANCHTOIP, LASTINTFROMIP,
> + LASTINTTOIP */
> +
> rdtscll(vcpu->host_tsc);
> put_cpu();
> }
> @@ -815,18 +829,16 @@
>
> static void load_host_msrs(struct kvm_vcpu *vcpu)
> {
> - int i;
> -
> - for ( i = 0; i < NR_HOST_SAVE_MSRS; i++)
> - wrmsrl(host_save_msrs[i], vcpu->svm->host_msrs[i]);
> +#ifdef CONFIG_X86_64
> + wrmsrl(MSR_GS_BASE, vcpu->svm->host_gs_base);
> +#endif
> }
>
> static void save_host_msrs(struct kvm_vcpu *vcpu)
> {
> - int i;
> -
> - for ( i = 0; i < NR_HOST_SAVE_MSRS; i++)
> - rdmsrl(host_save_msrs[i], vcpu->svm->host_msrs[i]);
> +#ifdef CONFIG_X86_64
> + rdmsrl(MSR_GS_BASE, vcpu->svm->host_gs_base);
> +#endif
> }
>
> static void new_asid(struct kvm_vcpu *vcpu, struct svm_cpu_data *svm_data)
> @@ -1498,6 +1510,11 @@
> load_db_regs(vcpu->svm->db_regs);
> }
>
> + if ((vcpu->svm->vmcb->save.dr7 & 0xff) ||
> + (vcpu->svm->host_dr7 & 0xff)) {
> + wrmsrl(MSR_IA32_DEBUGCTLMSR, vcpu->svm->host_ia32_debugctl);
> + }
> +
> if (vcpu->fpu_active) {
> fx_save(vcpu->host_fx_image);
> fx_restore(vcpu->guest_fx_image);
> @@ -1617,6 +1634,11 @@
> fx_restore(vcpu->host_fx_image);
> }
>
> + if ((vcpu->svm->vmcb->save.dr7 & 0xff) ||
> + (vcpu->svm->host_dr7 & 0xff)) {
> + rdmsrl(MSR_IA32_DEBUGCTLMSR, vcpu->svm->host_ia32_debugctl);
> + }
> +
> if ((vcpu->svm->vmcb->save.dr7 & 0xff))
> load_db_regs(vcpu->svm->host_db_regs);
>
> Index: kernel/drivers/kvm/kvm_svm.h
> ===================================================================
> --- kernel.orig/drivers/kvm/kvm_svm.h 2007-04-27 17:52:37.240774864 -0500
> +++ kernel/drivers/kvm/kvm_svm.h 2007-04-27 17:53:42.509852456 -0500
> @@ -9,17 +9,15 @@
> #include "svm.h"
> #include "kvm.h"
>
> -static const u32 host_save_msrs[] = {
> +static const u32 host_save_user_msrs[] = {
> #ifdef CONFIG_X86_64
> MSR_STAR, MSR_LSTAR, MSR_CSTAR, MSR_SYSCALL_MASK, MSR_KERNEL_GS_BASE,
> - MSR_FS_BASE, MSR_GS_BASE,
> + MSR_FS_BASE,
> #endif
> MSR_IA32_SYSENTER_CS, MSR_IA32_SYSENTER_ESP, MSR_IA32_SYSENTER_EIP,
> - MSR_IA32_DEBUGCTLMSR, /*MSR_IA32_LASTBRANCHFROMIP,
> - MSR_IA32_LASTBRANCHTOIP, MSR_IA32_LASTINTFROMIP,MSR_IA32_LASTINTTOIP,*/
> };
>
> -#define NR_HOST_SAVE_MSRS ARRAY_SIZE(host_save_msrs)
> +#define NR_HOST_SAVE_USER_MSRS ARRAY_SIZE(host_save_user_msrs)
> #define NUM_DB_REGS 4
>
> struct vcpu_svm {
> @@ -32,7 +30,9 @@
>
> u64 next_rip;
>
> - u64 host_msrs[NR_HOST_SAVE_MSRS];
> + u64 host_user_msrs[NR_HOST_SAVE_USER_MSRS];
> + u64 host_ia32_debugctl;
> + u64 host_gs_base;
> unsigned long host_cr2;
> unsigned long host_db_regs[NUM_DB_REGS];
> unsigned long host_dr6;
>
> ------------------------------------------------------------------------
>
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