Re: [PATCH 42/43] KVM: VMX: Drop VMWRITEs to zero fields at vCPU RESET

[Sean Christopherson]

On Mon, May 24, 2021, Jim Mattson wrote:
> On Mon, May 24, 2021 at 3:28 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> > 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.
> 
> The memset should probably be dropped, unless it is there to prevent
> information leakage. However, it is not necessary to VMWRITE all known
> (or possible) fields--just those that aren't guarded by an enable bit.

Yeah, I was thinking of defense-in-depth, e.g. better to have VM-Enter consume
'0' than random garbage because KVM botched an enabling sequence.  We essentially
get that today via the memset().  I'll fiddle with the sequence and see how much
overhead a paranoid and/or really paranoid approach would incur.