Re: [RFC] TDX module configurability of 0x80000008

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On 5/8/2024 1:11 AM, Sean Christopherson wrote:
On Wed, May 08, 2024, Xiaoyao Li wrote:
On 4/25/2024 12:55 AM, Edgecombe, Rick P wrote:
One of the TDX module features is called MAXPA_VIRT. In short, it is similar to
KVM’s allow_smaller_maxphyaddr. It requires an explicit opt-in by the VMM, and
allows a TD’s 0x80000008.EAX[7:0] to be configured by the VMM. Accesses to
physical addresses above the specified value by the TD will cause the TDX module
to inject a mostly correct #PF with the RSVD error code set. It has to deal with
the same problems as allow_smaller_maxphyaddr for correctly setting the RSVD
bit. I wasn’t thinking to push this feature for KVM due the movement away from
allow_smaller_maxphyaddr and towards 0x80000008.EAX[23:16].

I would like to get your opinion of the MAXPA_VIRT feature of TDX. What is
likely the KVM's decision on it? Won't support it due to it has the same
limitation of allow_smaller_maxphyaddr?

Not supporting MAXPA_VIRT has my vote.  I'm of the opinion that allow_smaller_maxphyaddr
should die a horrible, fiery death :-)

Thanks for the response. It's good to know your preference.

I'm not sure if there is any user of "allow_smaller_maxphyaddr". On QEMU side, it doesn't check it nor rely on it. QEMU always allow the user to configure a smaller PA.

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