On Mon, May 24, 2021, Paolo Bonzini wrote: > On 24/04/21 02:46, Sean Christopherson wrote: > > Don't waste time writing zeros via VMWRITE during vCPU RESET, the VMCS > > is zero allocated. > > Is this guaranteed to be valid, or could the VMCS in principle use some > weird encoding? (Like it does for the access rights, even though this does > not matter for this patch). Phooey. In principle, the CPU can do whatever it wants, e.g. the SDM states that software should never write to the data portion of the VMCS under any circumstance. In practice, I would be flabbergasted if Intel ever ships a CPU that doesn't play nice with zero initiazing the VMCS via software writes. I'd bet dollars to donuts that KVM isn't the only software that relies on that behavior. That said, I'm not against switching to VMWRITE for everything, but regardless of which route we choose, we should commit to one or the other. I.e. double down on memset() and bet that Intel won't break KVM, or replace the memset() in alloc_vmcs_cpu() with a sequence that writes all known (possible?) fields. The current approach of zeroing the memory in software but initializing _some_ fields is the worst option, e.g. I highly doubt vmcs01 and vmcs02 do VMWRITE(..., 0) on the same fields.
