linux-kernel - Re: [RFC v2-fix-v2 2/2] x86/tdx: Ignore WBINVD instruction for TDX guest

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Date:   Mon, 24 May 2021 17:50:37 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Raj Ashok <ashok.raj@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v2-fix-v2 2/2] x86/tdx: Ignore WBINVD instruction for TDX guest

On Mon, May 24, 2021 at 5:30 PM Kuppuswamy, Sathyanarayanan
<sathyanarayanan.kuppuswamy@...ux.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.