linux-kernel - Re: [RFC v2-fix-v4 1/1] x86/tdx: Skip WBINVD instruction for TDX guest

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Message-ID: <CAPcyv4jfFPGm2-cvPExeujZnaSKKky3AQRp69tzG1gcZ09dj8A@mail.gmail.com>
Date:   Tue, 8 Jun 2021 21:19:04 -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-v4 1/1] x86/tdx: Skip WBINVD instruction for TDX guest

On Tue, Jun 8, 2021 at 8:56 PM Kuppuswamy, Sathyanarayanan
<sathyanarayanan.kuppuswamy@...ux.intel.com> wrote:
>
>
>
> On 6/8/21 8:40 PM, Dan Williams wrote:
> > ..."KVM gets away with it" is not a justification that TDX can stand
> > on otherwise we would not be here fixing up ACPICA properly.
> >
> > How about:
> >
> > "TDX guests use standard ACPI mechanisms to signal sleep state entry
> > (including reboot) to the host. The ACPI specification mandates WBINVD
> > on any sleep state entry with the expectation that the platform is
> > only responsible for maintaining the state of memory over sleep
> > states, not preserving dirty data in any CPU caches. ACPI cache
> > flushing requirements pre-date the advent of virtualization. Given TDX
> > guest sleep state entry does not affect any host power rails it is not
> > required to flush caches. The host is responsible for maintaining
> > cache state over its own bare metal sleep state transitions that
> > power-off the cache. If the host fails to manage caches over its sleep
> > state transitions the guest..."
>
> >
> > I don't know how to finish the last sentence. What does TDX do if it
> > is resumed after host suspend and the host somehow arranged for dirty
> > TDX lines to be lost.
>
> TDX guest does not support S3. It will be disabled in ACPI tables. It
> is a TDX firmware spec requirement. Please check the following spec,
> sec 2.1
>
> https://software.intel.com/content/dam/develop/external/us/en/documents/tdx-virtual-firmware-design-guide-rev-1.pdf

I'm not asking about TDX guest entering S3...

>
> In TDX guest, we encounter cache flushes only in shutdown and reboot path.
> So there is no resume path.

Host is free to go into S3 independent of any guest state. A hostile
host is free to do just enough cache management so that it can resume
from S3 while arranging for TDX guest dirty data to be lost. Does a
TDX guest go fatal if the cache loses power?