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

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Date:   Thu, 10 Dec 2020 17:10:53 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Michael Roth <michael.roth@....com>
Cc:     Andy Lutomirski <luto@...nel.org>, kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.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 ML <x86@...nel.org>, "H . Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH] KVM: SVM: use vmsave/vmload for saving/restoring
 additional host state

> On Dec 10, 2020, at 4:48 PM, Michael Roth <michael.roth@....com> wrote:
>

>> I think there are two reasonable ways to do this:
>>
>> 1. VMLOAD before STGI.  This is obviously correct, and it's quite simple.
>
> For the testing I ended up putting it immediately after __svm_vcpu_run()
> since that's where we restored GS base previously. Does that seem okay or did
> you have another place in mind?

Looks okay.  If we get an NMI or MCE with the wrong MSR_GS_BASE, then
we are toast, at least on Zen 2 and earlier.  But that spot has GI ==
0, so this won't happen.

>
>>
>> 2. Save cpu_kernelmode_gs_base(cpu) before VM entry, and restore that
>> value to MSR_GS_BASE using code like this (or its asm equivalent)
>> before STGI:
>>
>> if (static_cpu_has(X86_FEATURE_FSGSBASE))
>>  wrgsbase(base);
>> else
>>  wrmsr...
>>
>> and then VMLOAD in the vcpu_put() path.
>>
>> I can't think of any reason to use loadsegment(), load_gs_index(), or
>> savesegment() at all, nor can I think of any reason to touch
>> MSR_KERNEL_GS_BASE or MSR_FS_BASE.
>
> I'm sort of lumping MSR_GS_BASE restoration in with everything else since I
> don't fully understand what the original was code doing either and was content
> to leave it be if we couldn't use VMLOAD to handle it without a performance
> regression, but since it looks like we can use VMLOAD here instead I agree
> we should just drop it all.

The original code is entirely bogus. Don’t try to hard to understand
how it’s correct — I’m pretty sure it’s not. In fact, I was planning
to write a patch a lot like yours, but I don’t have an SVM-capable CPU
to test on.  In general, I suspect that you could delete all these
fields from the structs, see what fails to compile, and fix it pretty
easily.

MSR_GS_BASE is kernel state. (On 32-bit, fs and maybe gs are kernel
state.). Everything else is host *user* state and isn’t touched by
normal kernel code.