linux-kernel - Re: [PATCH v3 2/4] x86/tdx: Add tdx_hcall_get

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Message-ID: <386efdc4a7f9dabd84cd19823c094a468a8644c4.camel@intel.com>
Date:   Tue, 19 Apr 2022 19:21:41 +1200
From:   Kai Huang <kai.huang@...el.com>
To:     Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Hans de Goede <hdegoede@...hat.com>,
        Mark Gross <mgross@...ux.intel.com>
Cc:     "H . Peter Anvin" <hpa@...or.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Andi Kleen <ak@...ux.intel.com>, linux-kernel@...r.kernel.org,
        platform-driver-x86@...r.kernel.org
Subject: Re: [PATCH v3 2/4] x86/tdx: Add tdx_hcall_get_quote() API support

On Mon, 2022-04-18 at 22:28 -0700, Sathyanarayanan Kuppuswamy wrote:
> 
> On 4/18/22 9:40 PM, Kai Huang wrote:
> > > Please check the latest 1.0 specification (updated on Feb 2022). It has
> > > details about R13 register.
> > Thanks.  So it seems GHCI 1.0 has also been updated and it is consistent with
> > GHCI 1.5 now.  In this case, why do we still assume 8K shared memory?  Are you
> > going to update the driver?
> > 
> 
> Since the GetQuote spec only requires memory in 4K alignment, we just
> went with 8k constant allocation. Since existing users does not
> require more than 8k, it works. But I agree that this needs to be
> changed.

Quote format can be vendor specific, so there's no guarantee 3rd party won't
have a Quote larger than 8k.

> 
> In next version, I will change the driver to choose the allocation size
> based on user space request. Since this change would require us to do
> the memory allocation in IOCTL routine (instead of init code), it will
> make it slower. But I think this is negligible compared to time it takes
> for Quote request. So it should be fine.
> 
> > 

Yes attestation request should never be something that is very frequent, thus
should never be in a performance critical path.