On 16/05/2024 11:20 am, Sean Christopherson wrote:
On Thu, May 16, 2024, Kai Huang wrote:
You had said up the thread, why not opt all non-normal VMs into the new
behavior. It will work great for TDX. But why do SEV and others want this
automatically?
Because I want flexibility in KVM, i.e. I want to take the opportunity to try and
break away from KVM's godawful ABI. It might be a pipe dream, as keying off the
VM type obviously has similar risks to giving userspace a memslot flag. The one
sliver of hope is that the VM types really are quite new (though less so for SEV
and SEV-ES), whereas a memslot flag would be easily applied to existing VMs.
Btw, does the "zap-leaf-only" approach always have better performance,
assuming we have to hold MMU write lock for that?
I highly doubt it, especially given how much the TDP MMU can now do with mmu_lock
held for read.
Consider a huge memslot being deleted/moved.
If we can always have a better performance for "zap-leaf-only", then instead
of letting userspace to opt-in this feature, we perhaps can do the opposite:
We always do the "zap-leaf-only" in KVM, but add a quirk for the VMs that
userspace know can have such bug and apply this quirk.
Hmm, a quirk isn't a bad idea. It suffers the same problems as a memslot flag,
i.e. who knows when it's safe to disable the quirk, but I would hope userspace
would be much, much cautious about disabling a quirk that comes with a massive
disclaimer.
Though I suspect Paolo will shoot this down too ;-)
The quirk only works based on the assumption that userspace _exactly_
knows what kinda VMs will have this bug.
But as mentioned above, the first step is we need to convince ourselves
that doing "zap-leaf-only" by default is the right thing to do.
:-)
