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

[Paolo Bonzini]

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.

Paolo

> "failures" that are squashed and never propagated to userspace (PV
> features tend to do this), page splits, etc.