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

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Date:   Wed, 02 Feb 2022 02:33:16 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...el.com,
        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,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Isaku Yamahata <isaku.yamahata@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCHv2 26/29] x86/tdx: ioapic: Add shared bit for IOAPIC base
 address

On Mon, Jan 24 2022 at 18:02, Kirill A. Shutemov wrote:

> From: Isaku Yamahata <isaku.yamahata@...el.com>
>
> The kernel interacts with each bare-metal IOAPIC with a special
> MMIO page. When running under KVM, the guest's IOAPICs are
> emulated by KVM.
>
> When running as a TDX guest, the guest needs to mark each IOAPIC
> mapping as "shared" with the host.  This ensures that TDX private
> protections are not applied to the page, which allows the TDX host
> emulation to work.
>
> Earlier patches in this series modified ioremap() so that

The concept of earlier patches does not exist.

> 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?

Thanks,

        tglx