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Date:   Tue, 25 May 2021 00:54:58 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Kirill Shutemov <kirill.shutemov@linux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@kernel.org>,
        Raj Ashok <ashok.raj@intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [RFC v2-fix-v2 2/2] x86/tdx: Ignore WBINVD instruction for TDX
 guest
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On Mon, May 24, 2021, Dan Williams wrote:
> On Mon, May 24, 2021 at 5:30 PM Kuppuswamy, Sathyanarayanan
> <sathyanarayanan.kuppuswamy@linux.intel.com> wrote:
> >
> >
> >
> > On 5/24/21 4:39 PM, Dan Williams wrote:
> > >> Functionally only DMA devices can notice a side effect from
> > >> WBINVD's cache flushing. But, TDX does not support DMA,
> > >> because DMA typically needs uncached access for MMIO, and
> > >> the current TDX module always sets the IgnorePAT bit, which
> > >> prevents that.
> >
> > > I thought we discussed that there are other considerations for wbinvd
> > > besides DMA? In any event this paragraph is actively misleading
> > > because it disregards ACPI and Persistent Memory secure-erase whose
> > > usages of wbinvd have nothing to do with DMA. I would much prefer a
> > > patch to shutdown all the known wbinvd users as a precursor to this
> > > patch rather than assuming it's ok to simply ignore it. You have
> > > mentioned that TDX does not need to use those paths, but rather than
> > > assume they can't be used why not do the audit to explicitly disable
> > > them? Otherwise this statement seems to imply that the audit has not
> > > been done.
> >
> > But KVM also emulates WBINVD only if DMA is supported. Otherwise it
> > will be treated as noop.
> >
> > static bool need_emulate_wbinvd(struct kvm_vcpu *vcpu)
> > {
> >          return kvm_arch_has_noncoherent_dma(vcpu->kvm);
> > }
> 
> That makes KVM also broken for the cases where wbinvd is needed, but
> it does not make the description of this patch correct.

Yep!  KVM has a long and dubious history of making things work for specific use
cases without stricitly adhering to the architecture.

KVM also has to worry about malicious/buggy guests, e.g. letting the guest do
WBINVD at will would be a massive noisy neighbor problem (at best), while
ratelimiting might unnecessarily harm legitimate use case.  I.e. KVM has a
somewhat sane reason for "emulating" WBINVD as a nop.

And FWIW, IIRC all modern hardware has a coherent IOMMU, though that could be me
making things up.