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Date:   Mon, 7 Feb 2022 08:08:15 -0600
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Subject: Re: [PATCHv2 26/29] x86/tdx: ioapic: Add shared bit for IOAPIC base
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To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>
Cc:     mingo@redhat.com, bp@alien8.de, dave.hansen@intel.com,
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References: <20220124150215.36893-1-kirill.shutemov@linux.intel.com>
 <20220124150215.36893-27-kirill.shutemov@linux.intel.com>
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From:   Tom Lendacky <thomas.lendacky@amd.com>
In-Reply-To: <20220204223153.l2g3lpm4hxknwsof@black.fi.intel.com>
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On 2/4/22 16:31, Kirill A. Shutemov wrote:
> 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?

Correct, FIXMAP_PAGE_NOCACHE => PAGE_KERNEL_IO_NOCACHE, which does not 
have the encryption bit set, so it is mapped as shared under SEV.

Thanks,
Tom

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