Re: [PATCH v3 4/4] platform/x86: intel_tdx_attest: Add TDX Guest attestation interface driver

From: Isaku Yamahata <isaku.yamahata@gmail.com>
To: Sathyanarayanan Kuppuswamy  <sathyanarayanan.kuppuswamy@linux.intel.com>
Cc: Isaku Yamahata <isaku.yamahata@gmail.com>,
	Kai Huang <kai.huang@intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, Hans de Goede <hdegoede@redhat.com>,
	Mark Gross <mgross@linux.intel.com>,
	"H . Peter Anvin" <hpa@zytor.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Tony Luck <tony.luck@intel.com>, Andi Kleen <ak@linux.intel.com>,
	linux-kernel@vger.kernel.org,
	platform-driver-x86@vger.kernel.org
Subject: Re: [PATCH v3 4/4] platform/x86: intel_tdx_attest: Add TDX Guest attestation interface driver
Date: Thu, 21 Apr 2022 09:53:31 -0700	[thread overview]
Message-ID: <20220421165331.GA1451512@private.email.ne.jp> (raw)
In-Reply-To: <a2c2b7f5-0af0-18a3-062b-c6c9f78daaad@linux.intel.com>

On Thu, Apr 21, 2022 at 07:53:39AM -0700,
Sathyanarayanan Kuppuswamy <sathyanarayanan.kuppuswamy@linux.intel.com> wrote:

> On 4/20/22 11:57 PM, Isaku Yamahata wrote:
> > On Wed, Apr 20, 2022 at 07:42:06PM -0700,
> > Sathyanarayanan Kuppuswamy <sathyanarayanan.kuppuswamy@linux.intel.com> wrote:
> > 
> >    TDG.VP.VMCALL<GetQuote> API allows one TD to issue multiple requests. It's
> >    implementation specific that how many concurrent requests are allowed. The TD
> >    should be able to handle TDG.VP.VMCALL_RETRY if it chooses to issue multiple
> >    requests simultaneously
> 
> Do you know why we should handle VMCALL_RETRY case? IIUC, as per
> above spec, if each request we send uses different GPA buffer, then we
> should not even worry about checking for IN_FLIGHT status. right?

Not correct.  User space  VMM, e.g. qemu, may return RETRY error for various
reasons even if different GPAs are used or even if only single request is issued
at the same time.  Other user space VMM (there are severals alternatives to qemu)
would support TDX in future. They would choose different way to implement.

Attestation driver should check IN_FLIGHT always before processing shared GPA.
Interrupt notifies only that it needs attention from attestation driver.  It
doesn't guarantee that IN_FLIGHT is cleared. Interrupt indicates only that the
state may be changed.  It may not be changed.  VMM inject the iterrupt
spuriously.
-- 
Isaku Yamahata <isaku.yamahata@gmail.com>