linux-kernel - Re: [PATCH v3 1/1] x86/tdx: Handle MWAIT, MONITOR and WBINVD

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Message-ID: <YGNYhWXe57FMm6Ku@google.com>
Date:   Tue, 30 Mar 2021 16:57:41 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Andi Kleen <ak@...ux.intel.com>, Andy Lutomirski <luto@...nel.org>,
        "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/1] x86/tdx: Handle MWAIT, MONITOR and WBINVD

On Tue, Mar 30, 2021, Andy Lutomirski wrote:
> 
> > On Mar 30, 2021, at 8:14 AM, Sean Christopherson <seanjc@...gle.com> wrote:
> > 
> > On Mon, Mar 29, 2021, Andy Lutomirski wrote:
> >> 
> >>>> On Mar 29, 2021, at 7:04 PM, Andi Kleen <ak@...ux.intel.com> wrote:
> >>> 
> >>> 
> >>>> 
> >>>>> No, if these instructions take a #VE then they were executed at CPL=0.  MONITOR
> >>>>> and MWAIT will #UD without VM-Exit->#VE.  Same for WBINVD, s/#UD/#GP.
> >>>> 
> >>>> Dare I ask about XSETBV?
> >>> 
> >>> XGETBV does not cause a #VE, it just works normally. The guest has full
> >>> AVX capabilities.
> >>> 
> >> 
> >> X *SET* BV
> > 
> > Heh, XSETBV also works normally, relative to the features enumerated in CPUID.
> > XSAVES/XRSTORS support is fixed to '1' in the virtual CPU model.  A subset of
> > the features managed by XSAVE can be hidden by the VMM, but attempting to enable
> > unsupported features will #GP (either from hardware or injected by TDX Module),
> > not #VE.
> 
> Normally in non-root mode means that every XSETBV results in a VM exit and,
> IIUC, there’s a buglet in that this happens even if CPL==3.  Does something
> special happen in TDX or does the exit get reflected back to the guest as a
> #VE?

Hmm, I forgot about that quirk.  I would expect the TDX Module to inject a #GP
for that case.  I can't find anything in the spec that confirms or denies that,
but injecting #VE would be weird and pointless.

Andi/Sathya, the TDX Module spec should be updated to state that XSETBV will
#GP at CPL!=0.  If that's not already the behavior, the module should probably
be changed...