Am 27/09/2022 um 17:58 schrieb Sean Christopherson: > On Tue, Sep 27, 2022, Emanuele Giuseppe Esposito wrote: >> >> Am 26/09/2022 um 23:28 schrieb Sean Christopherson: >>> On Mon, Sep 26, 2022, David Hildenbrand wrote: >>>> As Sean said "This is an awful lot of a complexity to take on for something >>>> that appears to be solvable in userspace." >>> >>> And if the userspace solution is unpalatable for whatever reason, I'd like to >>> understand exactly what KVM behavior is problematic for userspace. E.g. the >>> above RHBZ bug should no longer be an issue as the buggy commit has since been >>> reverted. >> >> It still is because I can reproduce the bug, as also pointed out in >> multiple comments below. > > You can reproduce _a_ bug, but it's obviously not the original bug, because the > last comment says: > > Second, indeed the patch was reverted and somehow accepted without generating > too much noise: > > ... > > The underlying issue of course as we both know is still there. > > You might have luck reproducing it with this bug > > https://bugzilla.redhat.com/show_bug.cgi?id=1855298 > > But for me it looks like it is 'working' as well, so you might have > to write a unit test to trigger the issue. > >>> If the issue is KVM doing something nonsensical on a code fetch to MMIO, then I'd >>> much rather fix _that_ bug and improve KVM's user exit ABI to let userspace handle >>> the race _if_ userspace chooses not to pause vCPUs. >>> >> >> Also on the BZ they all seem (Paolo included) to agree that the issue is >> non-atomic memslots update. > > Yes, non-atomic memslot likely results in the guest fetching from a GPA without a > memslot. I'm asking for an explanation of exactly what happens when that occurs, > because it should be possible to adjust KVM and/or QEMU to play nice with the > fetch, e.g. to resume the guest until the new memslot is installed, in which case > an atomic update isn't needed. > > I assume the issue is that KVM exits with KVM_EXIT_INTERNAL_ERROR because the > guest is running at CPL=0, and QEMU kills the guest in response. If that's correct, > then that problem can be solved by exiting to userspace with KVM_EXIT_MMIO instead > of KVM_EXIT_INTERNAL_ERROR so that userspace can do something sane in response to > the MMIO code fetch. > > I'm pretty sure this patch will Just Work for QEMU, because QEMU simply resumes > the vCPU if mmio.len==0. It's a bit of a hack, but I don't think it violates KVM's > ABI in any way, and it can even become "official" behavior since KVM x86 doesn't > otherwise exit with mmio.len==0. > > Compile tested only... So basically you are just making KVM catch the failed kvm_vcpu_read_guest_page() by retuning mmio.len = 0 to QEMU which basically ends up in doing nothing and retry again executing the instruction? I wonder if there are some performance implications in this, but it's definitely simpler than what I did. Tested on the same failing machine used for the BZ, fixes the bug. Do you want me to re-send the patch on your behalf (and add probably a small documentation on Documentation/virt/kvm/api.rst)? Emanuele > > --- > From: Sean Christopherson <seanjc@xxxxxxxxxx> > Date: Tue, 27 Sep 2022 08:16:03 -0700 > Subject: [PATCH] KVM: x86: Exit to userspace with zero-length MMIO "read" on > MMIO fetch > > Exit to userspace with KVM_EXIT_MMIO if emulation fails due to not being > able to fetch instruction bytes, e.g. if the resolved GPA isn't backed by > a memslot. If userspace is manipulating memslots without pausing vCPUs, > e.g. to emulate BIOS relocation, then a vCPU may fetch while there is no > valid memslot installed. Depending on guest context, KVM will either > exit to userspace with KVM_EXIT_INTERNAL_ERROR (L1, CPL=0) or simply > resume the guest (L2 or CPL>0), neither of which is desirable as exiting > with "emulation error" effectively kills the VM, and resuming the guest > doesn't provide userspace an opportunity to react the to fetch. > > Use "mmio.len == 0" to indicate "fetch". This is a bit of a hack, but > there is no other way to communicate "fetch" to userspace without > defining an entirely new exit reason, e.g. "mmio.is_write" is a boolean > and not a flag, and there is no known use case for actually supporting > code fetches from MMIO, i.e. there's no need to allow userspace to fill > in the instruction bytes. > > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> > --- > arch/x86/kvm/emulate.c | 2 ++ > arch/x86/kvm/kvm_emulate.h | 1 + > arch/x86/kvm/x86.c | 9 ++++++++- > 3 files changed, 11 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c > index f092c54d1a2f..e141238d93b0 100644 > --- a/arch/x86/kvm/emulate.c > +++ b/arch/x86/kvm/emulate.c > @@ -5353,6 +5353,8 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len, int > done: > if (rc == X86EMUL_PROPAGATE_FAULT) > ctxt->have_exception = true; > + if (rc == X86EMUL_IO_NEEDED) > + return EMULATION_IO_FETCH; > return (rc != X86EMUL_CONTINUE) ? EMULATION_FAILED : EMULATION_OK; > } > > diff --git a/arch/x86/kvm/kvm_emulate.h b/arch/x86/kvm/kvm_emulate.h > index 89246446d6aa..3cb2e321fcd2 100644 > --- a/arch/x86/kvm/kvm_emulate.h > +++ b/arch/x86/kvm/kvm_emulate.h > @@ -516,6 +516,7 @@ bool x86_page_table_writing_insn(struct x86_emulate_ctxt *ctxt); > #define EMULATION_OK 0 > #define EMULATION_RESTART 1 > #define EMULATION_INTERCEPTED 2 > +#define EMULATION_IO_FETCH 3 > void init_decode_cache(struct x86_emulate_ctxt *ctxt); > int x86_emulate_insn(struct x86_emulate_ctxt *ctxt); > int emulator_task_switch(struct x86_emulate_ctxt *ctxt, > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index aa5ab0c620de..7eb72694c601 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -7129,8 +7129,13 @@ static int kvm_fetch_guest_virt(struct x86_emulate_ctxt *ctxt, > bytes = (unsigned)PAGE_SIZE - offset; > ret = kvm_vcpu_read_guest_page(vcpu, gpa >> PAGE_SHIFT, val, > offset, bytes); > - if (unlikely(ret < 0)) > + if (unlikely(ret < 0)) { > + vcpu->run->mmio.phys_addr = gpa; > + vcpu->run->mmio.len = 0; > + vcpu->run->mmio.is_write = 0; > + vcpu->run->exit_reason = KVM_EXIT_MMIO; > return X86EMUL_IO_NEEDED; > + } > > return X86EMUL_CONTINUE; > } > @@ -8665,6 +8670,8 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, > r = x86_decode_emulated_instruction(vcpu, emulation_type, > insn, insn_len); > if (r != EMULATION_OK) { > + if (r == EMULATION_IO_FETCH) > + return 0; > if ((emulation_type & EMULTYPE_TRAP_UD) || > (emulation_type & EMULTYPE_TRAP_UD_FORCED)) { > kvm_queue_exception(vcpu, UD_VECTOR); > > base-commit: 39d9b48cc777bdf6d67d01ed24f1f89b13f5fbb2 >
