On Tue, Jan 04, 2022, Michael Roth wrote: > On Thu, Dec 30, 2021 at 09:11:12PM +0000, Sean Christopherson wrote: > > not in-kernel. That is bound to bite someone. The only issue with SEV is the > > address, not the VM-Exit mechanism. That doesn't change with SEV-ES, SEV-SNP, > > or TDX, as PIO and HLT will both get reflected as #VC/#VE, i.e. the guest side > > needs to be updated to use VMGEXIT/TDCALL no matter what, at which point having > > the hypercall request PIO emulation is just as easy as requesting HLT. > > I'm not aware of any #VC handling needed for HLT in the case of > SEV-ES/SEV-SNP. That was one of the reasons for the SEV tests using > this ucall implementation. Ah, you're right, HLT is an "automatic" exit and the CPU takes care of adjusting RIP. TDX is the only one that requires a hypercall. > Of course, at some point, we'd want full support for PIO/MMIO/etc. in the #VC > handler, but it's not something I'd planned on adding until after the SEV-SNP > tests, since it seems like we'd need to import a bunch of intruction decoding > code from elsewhere in the kernel, which is a lot of churn that's not > immediately necessary for getting at least some basic tests in place. Since > the HLT implementation is only 20 lines of code it seemed like a reasonable > stop-gap until we start getting more CoCo tests in place. But the in-kernel > APIC issue probably needs more consideration... > > Perhaps for *just* PIO, the intruction decoding can be open-coded so it > can be added to the initial #VC handler implementation, which would avoid the > need for HLT implementation. I'll take a look at that. PIO shouldn't require instruction decoding or a #VC handler. What I was thinking is that the guest in the selftest would make a direct #VMGEXIT/TDCALL to request PIO instead of executing an OUT. > > I also don't like having to differentiate between a "shared" and "regular" ucall. > > I kind of like having to explicitly pass the ucall object being used, but that > > puts undue burden on simple single-vCPU tests. > > I tried to avoid it, but I got hung up on that fact that pre-allocating > arrays/lists of ucall structs needs to be done for each VM, and so we'd > end up needing some way for a guest to identify which pool it's ucall > struct should be allocated from. But you've gotten around that by just > sync_global_to_guest()'ing for each pool at the time ucall_init() is > called, so the guest only ever sees it's particular pool. Then the switch > from writing GVA to writing GPA solves the translation problem. Nice. > > > > > The inability to read guest private memory is really the only issue, and that can > > be easily solved without completely revamping the ucall framework, and without > > having to update a huge pile of tests to make them place nice with private memory. > > I think the first 5 patches in this series are still relevant cleanups > vs. having a complete standalone ucall implementation for each arch, and Andrew > has also already started looking at other header cleanups related to > patch #1, so maybe Paolo would still like to queue those. Would also > provide a better starting point for having a centralized allocator for > the ucall structs, which you hinted at wanting below. > > But the subsequent patches that add the ucall_shared() interfaces should > probably be set aside for now in favor of your proposal. > > > > > This would also be a good opportunity to clean up the stupidity of tests having to > > manually call ucall_init(), drop the unused/pointless @arg from ucall_init(), and > > maybe even fix arm64's lurking landmine of not being SMP safe (the address is shared > > by all vCPUs). > > I thought you *didn't* want to update a huge pile of tests :) I suppose > it's unavoidable, since with your proposal, having something like ucall_init() > being called at some point is required, as opposed to the current > implementation where it is optional. Are you intending to have it be > called automatically by vm_create*()? Yeah, I was thinking it could be done at the lowest level vm_create() helper. We'll need to expand vm_create() (or add yet another layer to avoid modifying a pile of tests) to allow opting out of initializing ucall, e.g. sev_migrate_tests.c needs to create multiple concurrent VMs, but happily doesn't need ucall support. > > To reduce the burden on tests and avoid ordering issues with creating vCPUs, > > allocate a ucall struct for every possible vCPU when the VM is created and stuff > > the GPA of the struct in the struct itself so that the guest can communicate the > > GPA instead of the GVA. Then confidential VMs just need to make all structs shared. > > So a separate call like: > > ucall_make_shared(vm->ucall_list) > > ? Might need some good documentation/assertions to make sure it gets > called at the right place for confidential VMs, and may need some extra > hooks in SEV selftest implementation for switching from private to shared > after the memory has already been allocated, but seems reasonable. Again, I was thinking that it would be done unconditionally by ucall_init(), i.e. would be automatically handled by the selftest framework and would Just Work for individual tests. > > If all architectures have a way to access a vCPU ID, the ucall structs could be > > stored as a simple array. If not, a list based allocator would probably suffice. > > I think list allocator is nicer, generating #VCs for both the PIO and the > cpuid checks for vCPU lookup seems like a lot of extra noise to sift > through while debugging where an errant test is failing, and doesn't seem to > have any disadvantage vs. an array. Ah, right, I forgot that querying the vCPU ID would require a hypercall.
