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

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Message-ID: <95d1475e-fc30-89cf-19a7-4f125f5113d8@amd.com>
Date:   Mon, 7 Feb 2022 08:08:15 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>
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 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.
>