On Thu, May 20, 2021, Sean Christopherson wrote: > On Mon, May 17, 2021, Tom Lendacky wrote: > > On 5/14/21 6:06 PM, Peter Gonda wrote: > > > On Fri, May 14, 2021 at 1:22 PM Tom Lendacky <thomas.lendacky@xxxxxxx> wrote: > > >> > > >> Currently, an SEV-ES guest is terminated if the validation of the VMGEXIT > > >> exit code and parameters fail. Since the VMGEXIT instruction can be issued > > >> from userspace, even though userspace (likely) can't update the GHCB, > > >> don't allow userspace to be able to kill the guest. > > >> > > >> Return a #GP request through the GHCB when validation fails, rather than > > >> terminating the guest. > > > > > > Is this a gap in the spec? I don't see anything that details what > > > should happen if the correct fields for NAE are not set in the first > > > couple paragraphs of section 4 'GHCB Protocol'. > > > > No, I don't think the spec needs to spell out everything like this. The > > hypervisor is free to determine its course of action in this case. > > The hypervisor can decide whether to inject/return an error or kill the guest, > but what errors can be returned and how they're returned absolutely needs to be > ABI between guest and host, and to make the ABI vendor agnostic the GHCB spec > is the logical place to define said ABI. > > For example, "injecting" #GP if the guest botched the GHCB on #VMGEXIT(CPUID) is > completely nonsensical. As is, a Linux guest appears to blindly forward the #GP, > which means if something does go awry KVM has just made debugging the guest that > much harder, e.g. imagine the confusion that will ensue if the end result is a > SIGBUS to userspace on CPUID. > > There needs to be an explicit error code for "you gave me bad data", otherwise > we're signing ourselves up for future pain. More concretely, I think the best course of action is to define a new return code in SW_EXITINFO1[31:0], e.g. '2', with additional information in SW_EXITINFO2. In theory, an old-but-sane guest will interpret the unexpected return code as fatal to whatever triggered the #VMGEXIT, e.g. SIGBUS to userspace. Unfortunately Linux isn't sane because sev_es_ghcb_hv_call() assumes any non-'1' result means success, but that's trivial to fix and IMO should be fixed irrespective of where this goes.
