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Subject: Re: [RFC v2-fix-v2 2/2] x86/tdx: Ignore WBINVD instruction for TDX
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To:     Dan Williams <dan.j.williams@intel.com>
Cc:     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>,
        Sean Christopherson <seanjc@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
References: <37ad50ca-f568-4c62-56e2-9e9b1f34084c@linux.intel.com>
 <20210524233211.802033-1-sathyanarayanan.kuppuswamy@linux.intel.com>
 <20210524233211.802033-2-sathyanarayanan.kuppuswamy@linux.intel.com>
 <CAPcyv4jKY0rmewFnyL6My5-b+w8ANAwDY2tLXZk4CYKydoVbtg@mail.gmail.com>
From:   "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@linux.intel.com>
Message-ID: <b420a7af-5202-fee9-9e0b-39680d0cc9c8@linux.intel.com>
Date:   Mon, 24 May 2021 17:29:48 -0700
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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);
}


-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer