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Date:   Sat, 5 Feb 2022 01:31:53 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To:     Thomas Gleixner <tglx@linutronix.de>,
        Tom Lendacky <thomas.lendacky@amd.com>
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        x86@kernel.org, linux-kernel@vger.kernel.org,
        Isaku Yamahata <isaku.yamahata@intel.com>
Subject: Re: [PATCHv2 26/29] x86/tdx: ioapic: Add shared bit for IOAPIC base
 address
Message-ID: <20220204223153.l2g3lpm4hxknwsof@black.fi.intel.com>
References: <20220124150215.36893-1-kirill.shutemov@linux.intel.com>
 <20220124150215.36893-27-kirill.shutemov@linux.intel.com>
 <871r0mvyqr.ffs@tglx>
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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