Re: [PATCH] x86/svm: require VMSAVEvirt for nested virt

On 28.07.2026 09:16, Stephen Cheng wrote:
> On 27/07/2026 11:48, Teddy Astie wrote:
>> Had you considered improving the current implementation of virtualized
>> VMLOAD/VMSAVE ? AIUI, the way Xen currently implements this instruction
>> (by mapping the L1 VMCB then calling vmload/vmsave on it) is more of a
>> implementation choice than a requirement.
>>
>> A better (at least, likely safer) way to emulate such instructions (and
>> not relying on VMSAVEvirt) would be to copy parts of L1 VMCB from/to L0
>> VMCB and let Xen perform the appropriate vmload/vmsave on its own VMCBs
>> as usual.
> 
> You're right that it's an implementation choice rather than a hardware
> requirement.  On whether it's worth building, I'll defer to Ross's
> reply: given how limited our resources for nested virt are, and that
> AMD nested virt doesn't work today, dropping support for older parts
> isn't a functional regression.

But the functions you drop aren't needed just for older parts. They're also
going to be needed when the insn emulator hits these insns. This still is
in need of properly wiring up, but imo we better wouldn't delete code which
is going to be needed later on.

Jan