Re: [PATCH v2] KVM: SVM: use vmsave/vmload for saving/restoring additional host state

Andy Lutomirski
2020-12-14T22:45:22Z

> On Dec 14, 2020, at 2:02 PM, Michael Roth <michael.roth@amd.com> wrote:
>
> ﻿On Mon, Dec 14, 2020 at 11:38:23AM -0800, Sean Christopherson wrote:
>> +Andy, who provided a lot of feedback on v1.
>> On Mon, Dec 14, 2020, Michael Roth wrote:
>> Cc: Andy Lutomirski <luto@kernel.org>
>>> Suggested-by: Tom Lendacky <thomas.lendacky@amd.com>
>>> Signed-off-by: Michael Roth <michael.roth@amd.com>
>>> ---
>>> v2:
>>> * rebase on latest kvm/next
>>> * move VMLOAD to just after vmexit so we can use it to handle all FS/GS
>>> host state restoration and rather than relying on loadsegment() and
>>> explicit write to MSR_GS_BASE (Andy)
>>> * drop 'host' field from struct vcpu_svm since it is no longer needed
>>> for storing FS/GS/LDT state (Andy)
>>> ---
>>> arch/x86/kvm/svm/svm.c | 44 ++++++++++++++++--------------------------
>>> arch/x86/kvm/svm/svm.h | 14 +++-----------
>>> 2 files changed, 20 insertions(+), 38 deletions(-)
>>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>>> index 0e52fac4f5ae..fb15b7bd461f 100644
>>> --- a/arch/x86/kvm/svm/svm.c
>>> +++ b/arch/x86/kvm/svm/svm.c
>>> @@ -1367,15 +1367,19 @@ static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>>> vmcb_mark_all_dirty(svm->vmcb);
>>> }
>>> -#ifdef CONFIG_X86_64
>>> - rdmsrl(MSR_GS_BASE, to_svm(vcpu)->host.gs_base);
>>> -#endif
>>> - savesegment(fs, svm->host.fs);
>>> - savesegment(gs, svm->host.gs);
>>> - svm->host.ldt = kvm_read_ldt();
>>> -
>>> - for (i = 0; i < NR_HOST_SAVE_USER_MSRS; i++)
>>> + for (i = 0; i < NR_HOST_SAVE_USER_MSRS; i++) {
>>> rdmsrl(host_save_user_msrs[i], svm->host_user_msrs[i]);
>>> + }
>
> Hi Sean,
>
> Hopefully I've got my email situation sorted out now...
>
>> Unnecessary change that violates preferred coding style. Checkpatch explicitly
>> complains about this.
>> WARNING: braces {} are not necessary for single statement blocks
>> #132: FILE: arch/x86/kvm/svm/svm.c:1370:
>> + for (i = 0; i < NR_HOST_SAVE_USER_MSRS; i++) {
>> rdmsrl(host_save_user_msrs[i], svm->host_user_msrs[i]);
>> +
>
> Sorry, that was an artifact from an earlier version of the patch that I
> failed to notice. I'll make sure to run everything through checkpatch
> going forward.
>
>>> +
>>> + asm volatile(__ex("vmsave")
>>> + : : "a" (page_to_pfn(sd->save_area) << PAGE_SHIFT)
>> I'm pretty sure this can be page_to_phys().
>>> + : "memory");
>> I think we can defer this until we're actually planning on running the guest,
>> i.e. put this in svm_prepare_guest_switch().
>
> One downside to that is that we'd need to do the VMSAVE on every
> iteration of vcpu_run(), as opposed to just once when we enter from
> userspace via KVM_RUN. It ends up being a similar situation to Andy's
> earlier suggestion of moving VMLOAD just after vmexit, but in that case
> we were able to remove an MSR write to MSR_GS_BASE, which cancelled out
> the overhead, but in this case I think it could only cost us extra.

If you want to micro-optimize, there is a trick you could play: use WRGSBASE if available. If X86_FEATURE_GSBASE is available, you could use WRGSBASE to restore GSBASE and defer VMLOAD to vcpu_put(). This would need benchmarking on Zen 3 to see if it’s worthwhile.