Re: [PATCH] KVM: x86: Suppress MMIO that is triggered during task switch emulation

[Tao Su]

On Fri, Jul 12, 2024 at 07:48:41AM -0700, Sean Christopherson wrote:

[...]

> See commit 0dc902267cb3 ("KVM: x86: Suppress pending MMIO write exits if
> emulator detects exception") for more details on KVM's limitations with
> respect to emulated MMIO during complex emulator flows.
> 

I try to understand the changelog of commit 0dc902267cb3 but I’m confused with
the MMIO read. The commit said, "For MMIO reads, KVM immediately exits to
userspace upon detecting MMIO as userspace provides the to-be-read value in a
buffer, and so KVM can safely (more or less) restart the instruction from the
beginning." But in read_emulated(), mc->end is adjusted after checking rc,
i.e., although the value will be saved in the buffer, mc->end is not adjusted
after existing to userspace.

Maybe this would really support a buffer for multiple MMIO read instructions
(e.g. POPA)?

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 5d4c86133453..841d5b6f21b0 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -1367,8 +1367,11 @@ static int read_emulated(struct x86_emulate_ctxt *ctxt,
 
 	rc = ctxt->ops->read_emulated(ctxt, addr, mc->data + mc->end, size,
 				      &ctxt->exception);
-	if (rc != X86EMUL_CONTINUE)
+	if (rc != X86EMUL_CONTINUE) {
+		if (rc == X86EMUL_IO_NEEDED)
+			mc->end += size;
 		return rc;
+	}
 
 	mc->end += size;

[...]