linux-kernel - Re: [PATCHv4 12/30] x86/tdx: Detect TDX at early kernel decompression time

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Date:   Thu, 24 Feb 2022 12:44:00 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        luto@...nel.org, peterz@...radead.org
Cc:     sathyanarayanan.kuppuswamy@...ux.intel.com, aarcange@...hat.com,
        ak@...ux.intel.com, dan.j.williams@...el.com, david@...hat.com,
        hpa@...or.com, jgross@...e.com, jmattson@...gle.com,
        joro@...tes.org, jpoimboe@...hat.com, knsathya@...nel.org,
        pbonzini@...hat.com, sdeep@...are.com, seanjc@...gle.com,
        tony.luck@...el.com, vkuznets@...hat.com, wanpengli@...cent.com,
        thomas.lendacky@....com, brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4 12/30] x86/tdx: Detect TDX at early kernel decompression
 time

On 2/24/22 07:56, Kirill A. Shutemov wrote:
> From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
> 
> The early decompression code does port I/O for its console output. But,
> handling the decompression-time port I/O demands a different approach
> from normal runtime because the IDT required to support #VE based port
> I/O emulation is not yet set up. Paravirtualizing I/O calls during
> the decompression step is acceptable because the decompression code size is
> small enough and hence patching it will not bloat the image size a lot.

It's not the *decompression* code size that matters.  It's that there
aren't a lot of call sites to the I/O instructions.  Right?

> To support port I/O in decompression code, TDX must be detected before
> the decompression code might do port I/O. Detect whether the kernel runs
> in a TDX guest.
> 
> Add an early_is_tdx_guest() interface to query the cached TDX guest
> status in the decompression code.

Nit: I was a bit surprised by the minor cpuid() munging and the new
shared/tdx.h header.  They look sane, but you can reduce reviewer
surprise by adding a sentence or two of changelog material.

> The actual port I/O paravirtualization will come later in the series.
> 
> Reviewed-by: Tony Luck <tony.luck@...el.com>
> Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

Acked-by: Dave Hansen <dave.hansen@...ux.intel.com>