linux-kernel - Re: [PATCHv2 26/29] x86/tdx: ioapic: Add shared bit for IOAPIC base address

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Date:   Sat, 5 Feb 2022 01:31:53 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Tom Lendacky <thomas.lendacky@....com>
Cc:     mingo@...hat.com, bp@...en8.de, dave.hansen@...el.com,
        luto@...nel.org, peterz@...radead.org,
        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,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Isaku Yamahata <isaku.yamahata@...el.com>
Subject: Re: [PATCHv2 26/29] x86/tdx: ioapic: Add shared bit for IOAPIC base
 address

On Wed, Feb 02, 2022 at 02:33:16AM +0100, Thomas Gleixner wrote:
> On Mon, Jan 24 2022 at 18:02, Kirill A. Shutemov wrote:
> > ioremap()-created mappings such as virtio will be marked as
> > shared. However, the IOAPIC code does not use ioremap() and instead
> > uses the fixmap mechanism.
> >
> > Introduce a special fixmap helper just for the IOAPIC code.  Ensure
> > that it marks IOAPIC pages as "shared".  This replaces
> > set_fixmap_nocache() with __set_fixmap() since __set_fixmap()
> > allows custom 'prot' values.
> 
> Why is this a TDX only issue and SEV does not suffer from that?

Hm. Good question.

I think it is because FIXMAP_PAGE_NOCACHE does not have __ENC bit set so
the mapping is accessible to host. With TDX the logic is oposit:
everything is private if the bit is not set.

Tom, does it sound right?

BTW, I will drop 'if (cc_platform_has(CC_ATTR_GUEST_TDX))'.
pgprot_decrypted() is nop on AMD in this case.

-- 
 Kirill A. Shutemov