Re: [PATCH 2/3] KVM: x86: Add support for VMware guest specific hypercalls

[Sean Christopherson]

On Mon, Feb 03, 2025, Paolo Bonzini wrote:
> On 2/3/25 20:41, Sean Christopherson wrote:
> > -EFAULT isn't the problem, KVM not being able to return useful information in
> > all situations is the issue.
> 
> Yes, that's why I don't want it to be an automatically opted-in API.  If
> incremental improvements are possible, it may be useful to allow interested
> userspace to enable it early.  For example...
> 
> > Specifically, "guest" accesses that are emulated
> > by KVM are problematic, because the -EFAULT from e.g. __kvm_write_guest_page()
> > is disconnected from the code that actually kicks out to userspace.  In that
> > case, userspace will get KVM_EXIT_MMIO, not -EFAULT.  There are more problems
> > beyond KVM_EXIT_MMIO vs. -EFAULT, e.g. instructions that perform multiple memory
> > accesses,
> 
> those are obviously synchronous and I expect VMware to handle them already.
> 
> That said my preferred solution to just use userfaultfd, which is
> synchronous by definition.

Oh, right, userfaultfd would be far better than piggybacking write-tracking.