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

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Date:   Mon, 14 Dec 2020 14:29:46 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Michael Roth <michael.roth@....com>
Cc:     Sean Christopherson <seanjc@...gle.com>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        linux-kernel@...r.kernel.org,
        Tom Lendacky <thomas.lendacky@....com>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v2] KVM: SVM: use vmsave/vmload for saving/restoring additional host state



> On Dec 14, 2020, at 2:02 PM, Michael Roth <michael.roth@....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@...nel.org>
>>> Suggested-by: Tom Lendacky <thomas.lendacky@....com>
>>> Signed-off-by: Michael Roth <michael.roth@....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.