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

[Thomas Gleixner]

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

> From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
>
> 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