linux-kernel - RE: [PATCH RESEND v9 1/2] x86/tdx: Retry TDVMCALL_MAP

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Message-ID: <SA1PR21MB133508E0683D75FE281E6BB4BF10A@SA1PR21MB1335.namprd21.prod.outlook.com>
Date:   Fri, 11 Aug 2023 19:04:49 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "arnd@...db.de" <arnd@...db.de>, "bp@...en8.de" <bp@...en8.de>,
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        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "jane.chu@...cle.com" <jane.chu@...cle.com>,
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        KY Srinivasan <kys@...rosoft.com>,
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CC:     "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
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Subject: RE: [PATCH RESEND v9 1/2] x86/tdx: Retry TDVMCALL_MAP_GPA() when
 needed

> From: Dave Hansen <dave.hansen@...el.com>
> Sent: Friday, August 11, 2023 7:27 AM
>  [...]
> On 8/10/23 19:12, Dexuan Cui wrote:
> > GHCI spec for TDX 1.0 says that the MapGPA call may fail with the R10
> > error code = TDG.VP.VMCALL_RETRY (1), and the guest must retry this
> > operation for the pages in the region starting at the GPA specified
> > in R11.
> >
> > When a fully enlightened TDX guest runs on Hyper-V, Hyper-V can return
> > the retry error when set_memory_decrypted() is called to decrypt up to
> > 1GB of swiotlb bounce buffers.
> 
> This changelog is not great.  It gives zero background and wastes bytes
> on telling me which register the error code is in (I don't care in a
> changelog) and then using marketing fluff words like "fully enlightened".
> 
> Let's stick to the facts, give some background, and also avoid
> regurgitating the GHCI, eh?
> 
> How's this?

I appreciate the great changelog! Will use this in v10.

> x86/tdx: Retry partially-completed page conversion hypercalls
> 
> TDX guest memory is private by default and the VMM may not access it.
> However, in cases where the guest needs to share data with the VMM,
> the guest and the VMM can coordinate to make memory shared between
> them.
> 
> The guest side of this protocol includes the "MapGPA" hypercall.  This
> call takes a guest physical address range.  The hypercall spec (aka.
> the GHCI) says that the MapGPA call is allowed to return partial
> progress in mapping this range and indicate that fact with a special
> error code.  A guest that sees such partial progress is expected to
> retry the operation for the portion of the address range that was not
> completed.
> 
> Hyper-V does this partial completion dance when set_memory_decrypted()
> is called to "decrypt" swiotlb bounce buffers that can be up to 1GB
> in size.  It is evidently the only VMM that does this, which is why
> nobody noticed this until now.

This clarifies the obscure "when needed" in my version by emphasizing
the "partial progress". This also gives necessary background info. Thanks!