KVM: x86: Trace unsatisfied MMIO reads on a per-page basis

Invoke the "unsatisfied MMIO reads" when KVM first detects that a
particular access "chunk" requires an exit to userspace instead of tracing
the entire access at the time KVM initiates the exit to userspace.  I.e.
precisely trace the first and/or second fragments of a page split instead
of tracing the entire access, as the GPA could be wrong on a page split
case.

Leave the completion tracepoint alone, at least for now, as fixing the
completion path would incur significantly complexity to track exactly which
fragment(s) of the overall access actually triggered MMIO, but add a
comment that the tracing for completed reads in is technically wrong.

Tested-by: Tom Lendacky <thomas.lendacky@gmail.com>
Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Link: https://patch.msgid.link/20260225012049.920665-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b0bb9cd9df8ac227eacb2a1075c34aa8f2bf0f4d..e2cf5349d5939e0bc59994ce41b2f0f07c4e5ca6 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7807,6 +7807,9 @@ static int vcpu_mmio_read(struct kvm_vcpu *vcpu, gpa_t addr, int len, void *v)
                v += n;
        } while (len);
 
+       if (len)
+               trace_kvm_mmio(KVM_TRACE_MMIO_READ_UNSATISFIED, len, addr, NULL);
+
        return handled;
 }
 
@@ -8138,7 +8141,6 @@ static int write_mmio(struct kvm_vcpu *vcpu, gpa_t gpa, int bytes, void *val)
 static int read_exit_mmio(struct kvm_vcpu *vcpu, gpa_t gpa,
                          void *val, int bytes)
 {
-       trace_kvm_mmio(KVM_TRACE_MMIO_READ_UNSATISFIED, bytes, gpa, NULL);
        return X86EMUL_IO_NEEDED;
 }
 
@@ -8246,6 +8248,11 @@ static int emulator_read_write(struct x86_emulate_ctxt *ctxt,
         * is copied to the correct chunk of the correct operand.
         */
        if (!ops->write && vcpu->mmio_read_completed) {
+               /*
+                * For simplicity, trace the entire MMIO read in one shot, even
+                * though the GPA might be incorrect if there are two fragments
+                * that aren't contiguous in the GPA space.
+                */
                trace_kvm_mmio(KVM_TRACE_MMIO_READ, bytes,
                               vcpu->mmio_fragments[0].gpa, val);
                vcpu->mmio_read_completed = 0;