linux-kernel - Re: [PATCH] KVM: x86: Ignore MSR_AMD64_BU

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Date:   Wed, 4 Oct 2023 17:10:31 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Borislav Petkov <bp@...en8.de>, kvm@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH] KVM: x86: Ignore MSR_AMD64_BU_CFG access

On Mon, Oct 02, 2023, Maciej S. Szmigiero wrote:
> On 26.09.2023 00:25, Tom Lendacky wrote:
> > > > It's partially documented in various AMD BKDGs, however I couldn't find
> > > > any definition for this particular bit (8) - other than that it is reserved.
> > > 
> > > I found it as MSR_AMD64_BU_CFG for Model 16h, but that's Jaguar/Puma, not Zen1.
> > > My guess is that Windows is trying to write this thing:
> > > 
> > >    MSRC001_1023 [Table Walker Configuration] (Core::X86::Msr::TW_CFG)
> > >    Read-write. Reset: 0000_0000_0000_0000h.
> > >    _lthree0_core[3,1]; MSRC001_1023
> > > 
> > >    Bits   Description
> > >    63:50  Reserved.
> > >    49     TwCfgCombineCr0Cd: combine CR0_CD for both threads of a core. Read-write. Reset: 0. Init: BIOS,1.
> > >           1=The host Cr0_Cd values from the two threads are OR'd together and used by both threads.
> > >    48:0   Reserved.
> > > 
> > > Though that still doesn't explain bit 8...  Perhaps a chicken-bit related to yet
> > > another speculation bug?
> > > 
> > > Boris or Tom, any idea what Windows is doing?  I doubt it changes our options in
> > > terms of "fixing" this in KVM, but having a somewhat accurate/helpful changelog
> > > would be nice.
> > 
> > It's definitely not related to a speculation bug, but I'm unsure what was
> > told to Microsoft that has them performing that WRMSR. The patch does the
> > proper thing, though, as a guest shouldn't be updating that setting.
> > 
> > And TW_CFG is the proper name of that MSR for Zen.
> 
> So, should I prepare v2 with MSR_AMD64_BU_CFG -> MSR_AMD64_TW_CFG change?

If we can get Paolo's attention, I'd like to get his thoughts on punting this
to QEMU/userspace.  I'm worried that "handling" uarch specific MSRs in KVM is
going to paint us into a corner and force KVM to check guest F/M/S someday, which
I want to avoid at pretty much all costs.