Re: [kvm-devel] [PATCH][SVM] Only save/restore MSRs when needed
Avi Kivity wrote: > Anthony Liguori wrote: > >> 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. >> >> > > Some observations from grepping the sources: > > - The host never uses the DEBUGCTL msr. So we only need to restore it > (to zero) if the guest changes it. > - The guest never uses the DEBUGCTL msr, as kvm doesn't handle wrmsr to > DEBUGCTL. > > So, doing nothing (in hardware and software) would probably work well > and be quite fast. If we ever emulate DEBUGCTL, we can conditionally > enable hardware or software save/restore (this is fairly similar to lazy > fpu). Okay, so I'll resubmit with DEBUGCTL disabled (including lbr_enable = 0). I'll also see if I can find something that uses DEBUGCTL. Regards, Anthony Liguori > If Linux ever gets DEBUGCTL support, we'd just ask for a hook so > we can conditionally save and restore on the host side. > > > - This SF.net email is sponsored by DB2 Express Download DB2 Express C - the FREE version of DB2 express and take control of your XML. No limits. Just data. Click to get it now. http://sourceforge.net/powerbar/db2/ ___ kvm-devel mailing list kvm-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kvm-devel
Re: [kvm-devel] [PATCH][SVM] Only save/restore MSRs when needed
Anthony Liguori wrote: > 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. > Some observations from grepping the sources: - The host never uses the DEBUGCTL msr. So we only need to restore it (to zero) if the guest changes it. - The guest never uses the DEBUGCTL msr, as kvm doesn't handle wrmsr to DEBUGCTL. So, doing nothing (in hardware and software) would probably work well and be quite fast. If we ever emulate DEBUGCTL, we can conditionally enable hardware or software save/restore (this is fairly similar to lazy fpu). If Linux ever gets DEBUGCTL support, we'd just ask for a hook so we can conditionally save and restore on the host side. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic. - This SF.net email is sponsored by DB2 Express Download DB2 Express C - the FREE version of DB2 express and take control of your XML. No limits. Just data. Click to get it now. http://sourceforge.net/powerbar/db2/ ___ kvm-devel mailing list kvm-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kvm-devel
Re: [kvm-devel] [PATCH][SVM] Only save/restore MSRs when needed
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 <[EMAIL PROTECTED]>
> 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 <[EMAIL PROTECTED]>
>
> 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_B
