On Thu, Apr 25, 2024, Michael Roth wrote: > On Wed, Apr 24, 2024 at 01:59:48PM -0700, Sean Christopherson wrote: > > On Sun, Apr 21, 2024, Michael Roth wrote: > > > +static int snp_begin_psc_msr(struct kvm_vcpu *vcpu, u64 ghcb_msr) > > > +{ > > > + u64 gpa = gfn_to_gpa(GHCB_MSR_PSC_REQ_TO_GFN(ghcb_msr)); > > > + u8 op = GHCB_MSR_PSC_REQ_TO_OP(ghcb_msr); > > > + struct vcpu_svm *svm = to_svm(vcpu); > > > + > > > + if (op != SNP_PAGE_STATE_PRIVATE && op != SNP_PAGE_STATE_SHARED) { > > > + set_ghcb_msr(svm, GHCB_MSR_PSC_RESP_ERROR); > > > + return 1; /* resume guest */ > > > + } > > > + > > > + vcpu->run->exit_reason = KVM_EXIT_VMGEXIT; > > > + vcpu->run->vmgexit.type = KVM_USER_VMGEXIT_PSC_MSR; > > > + vcpu->run->vmgexit.psc_msr.gpa = gpa; > > > + vcpu->run->vmgexit.psc_msr.op = op; > > > > Argh, no. > > > > This is the same crud that TDX tried to push[*]. Use KVM's existing user exits, > > and extend as *needed*. There is no good reason page state change requests need > > *two* exit reasons. The *only* thing KVM supports right now is private<=>shared > > conversions, and that can be handled with either KVM_HC_MAP_GPA_RANGE or > > KVM_EXIT_MEMORY_FAULT. > > > > The non-MSR flavor can batch requests, but I'm willing to bet that the overwhelming > > majority of requests are contiguous, i.e. can be combined into a range by KVM, > > and that handling any outliers by performing multiple exits to userspace will > > provide sufficient performance. > > That does tend to be the case. We won't have as much granularity with > the per-entry error codes, but KVM_SET_MEMORY_ATTRIBUTES would be > expected to be for the entire range anyway, and if that fails for > whatever reason then we KVM_BUG_ON() anyway. We do have to have handling > for cases where the entries aren't contiguous however, which would > involve multiple KVM_EXIT_HYPERCALLs until everything is satisfied. But > not a huge deal since it doesn't seem to be a common case. If it was less complex overall, I wouldn't be opposed to KVM marshalling everything into a buffer, but I suspect it will be simpler to just have KVM loop until the PSC request is complete. > KVM_HC_MAP_GPA_RANGE seems like a nice option because we'd also have the > flexibility to just issue that directly within a guest rather than > relying on SNP/TDX specific hcalls. I don't know if that approach is > practical for a real guest, but it could be useful for having re-usable > guest code in KVM selftests that "just works" for all variants of > SNP/TDX/sw-protected. (though we'd still want stuff that exercises > SNP/TDX->KVM_HC_MAP_GPA_RANGE translation). > > I think we'd there is some potential baggage there with the previous SEV > live migration use cases. There's some potential that existing guest kernels > will use it once it gets advertised and issue them alongside GHCB-based > page-state changes. It might make sense to use one of the reserved bits > to denote this flavor of KVM_HC_MAP_GPA_RANGE as being for > hardware/software-protected VMs and not interchangeable with calls that > were used for SEV live migration stuff. I don't think I follow, what exactly wouldn't be interchangeable, and why? > If this seems reasonable I'll give it a go and see what it looks like. > > > > > And the non-MSR version that comes in later patch is a complete mess. It kicks > > the PSC out to userspace without *any* validation. As I complained in the TDX > > thread, that will create an unmaintable ABI for KVM. > > > > KVM needs to have its own, well-defined ABI. Splitting functionality between > > KVM and userspace at seemingly random points is not maintainable. > > > > E.g. if/when KVM supports UNSMASH, upgrading to the KVM would arguably break > > userspace as PSC requests that previously exited would suddenly be handled by > > KVM. Maybe. It's impossible to review this because there's no KVM ABI, KVM is > > little more than a dumb pipe parroting information to userspace. > > It leans on the GHCB spec to avoid re-inventing structs/documentation > for things like Page State Change buffers, but do have some control > as we want over how much we farm out versus lock into the KVM ABI. For > instance the accompanying Documentation/ update mentions we only send a > subset of GHCB requests that need to be handled by userspace, so we > could handle SMASH/UNSMASH in KVM without breaking expectations (or if > SMASH/UNSMASH were intermixed with PSCs, documentation that only PSC > opcodes could be updated by userspace). > > But I'm certainly not arguing it wouldn't be better to have a > guest-agnostic alternative if we can reach an agreement on that, and > KVM_HC_MAP_GPA_RANGE seems like it could work. Yeah, I want to at least _try_ to achieve common ground, because the basic functionality of all this stuff is the exact same.