linux-kernel - Re: [PATCH v10 1/2] x86/tdx: Retry partially-completed page conversion hypercalls

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Message-ID: <20230814190355.GA2672897@ls.amr.corp.intel.com>
Date:   Mon, 14 Aug 2023 12:03:55 -0700
From:   Isaku Yamahata <isaku.yamahata@...il.com>
To:     Dexuan Cui <decui@...rosoft.com>
Cc:     x86@...nel.org, ak@...ux.intel.com, arnd@...db.de, bp@...en8.de,
        brijesh.singh@....com, dan.j.williams@...el.com,
        dave.hansen@...el.com, dave.hansen@...ux.intel.com,
        haiyangz@...rosoft.com, hpa@...or.com, jane.chu@...cle.com,
        kirill.shutemov@...ux.intel.com, kys@...rosoft.com,
        luto@...nel.org, mingo@...hat.com, peterz@...radead.org,
        rostedt@...dmis.org, sathyanarayanan.kuppuswamy@...ux.intel.com,
        seanjc@...gle.com, tglx@...utronix.de, tony.luck@...el.com,
        wei.liu@...nel.org, Jason@...c4.com, nik.borisov@...e.com,
        mikelley@...rosoft.com, linux-hyperv@...r.kernel.org,
        linux-kernel@...r.kernel.org, Tianyu.Lan@...rosoft.com,
        rick.p.edgecombe@...el.com, andavis@...hat.com, mheslin@...hat.com,
        vkuznets@...hat.com, xiaoyao.li@...el.com, isaku.yamahata@...il.com
Subject: Re: [PATCH v10 1/2] x86/tdx: Retry partially-completed page
 conversion hypercalls

On Fri, Aug 11, 2023 at 02:48:25PM -0700,
Dexuan Cui <decui@...rosoft.com> wrote:

> 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.

Now TDX KVM + TDX qemu supports partial completion because TD guest can pass
very large range. e.g. 1GB order.  I tested this patch with (patched) TDX
KVM/qemu.

Reviewed-by: Isaku Yamahata <isaku.yamahata@...el.com>
Tested-by: Isaku Yamahata <isaku.yamahata@...el.com>
-- 
Isaku Yamahata <isaku.yamahata@...il.com>