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Subject: Re: [RFC v2-fix-v2 2/2] x86/tdx: Ignore WBINVD instruction for TDX
 guest
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>,
        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>
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From:   Andi Kleen <ak@linux.intel.com>
Message-ID: <6f44fbeb-a8be-d2e4-5161-d46ddf09482e@linux.intel.com>
Date:   Mon, 24 May 2021 20:27:05 -0700
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On 5/24/2021 7:49 PM, Dan Williams wrote:
> On Mon, May 24, 2021 at 7:13 PM Andi Kleen <ak@linux.intel.com> wrote:
> [..]
>>> ...to explicitly error out a wbinvd use case before data is altered
>>> and wbinvd is needed.
>> I don't see any point of all of this. We really just want to be the same
>> as KVM. Not get into the business of patching a bazillion sub systems
>> that cannot be used in TDX anyways.
> Please let's not start this patch off with dubious claims of safety
> afforded by IgnorePAT. Instead make the true argument that wbinvd is
> known to be problematic in guests

That's just another reason to not support WBINVD, but I don't think it's 
the main reason. The main reason is that it is simply not needed, unless 
you do DMA in some form.

(and yes I consider direct mapping of persistent memory with a complex 
setup procedure a form of DMA -- my guess is that the reason that it 
works in KVM is that it somehow activates the DMA code paths in KVM)

IMNSHO that's the true reason.

> and for that reason many bare metal
> use cases that require wbinvd have not been ported to guests (like
> PMEM unlock), and others that only use wbinvd to opportunistically
> enforce a cache state (like ACPI sleep states)

ACPI sleep states are not supported or needed in virtualization. They 
are mostly obsolete on real hardware too.


> do not see ill effects
> from missing wbinvd. Given KVM ships with a policy to elide wbinvd in
> many scenarios adopt the same policy for TDX guests.