[Xen-devel] [PATCH v2] x86/hvm: fix interaction between internal and external emulation

[Paul Durrant <paul.durrant@xxxxxxxxxx>]

A call to handle_hvm_io_completion() is needed for completing I/O
that requires external emulation. Such completion should be requested when
hvm_vcpu_io_need_completion() returns true after hvm_emulate_once() has
completed. This is indicative of the underlying I/O emulation having
returned X86EMUL_RETRY and hence a re-emulation of the instruction is
needed to pick up the result of the I/O.

A call to handle_hvm_io_completion() is NOT needed when the underlying
I/O has not returned X86EMUL_RETRY since there will be no result to pick
up. Hence it bogus to request such completion when mmio_retry is set,
since this can only happen if the underlying I/O emulation has returned
X86EMUL_OKAY (meaning the I/O has completed successfully).

Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---

v2:
 - Re-worked Jan's original patch to avoid wrongly requesting the
   completion in the first place. (Commit comment changed accordingly).
---
 xen/arch/x86/hvm/io.c           | 2 +-
 xen/arch/x86/hvm/vmx/realmode.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/hvm/io.c b/xen/arch/x86/hvm/io.c
index e449b4196e..9d9e1b0e40 100644
--- a/xen/arch/x86/hvm/io.c
+++ b/xen/arch/x86/hvm/io.c
@@ -88,7 +88,7 @@ bool hvm_emulate_one_insn(hvm_emulate_validate_t *validate, 
const char *descr)
 
     rc = hvm_emulate_one(&ctxt);
 
-    if ( hvm_vcpu_io_need_completion(vio) || vio->mmio_retry )
+    if ( hvm_vcpu_io_need_completion(vio) )
         vio->io_completion = HVMIO_mmio_completion;
     else
         vio->mmio_access = (struct npfec){};
diff --git a/xen/arch/x86/hvm/vmx/realmode.c b/xen/arch/x86/hvm/vmx/realmode.c
index 03dea6c0fc..11211c8cd8 100644
--- a/xen/arch/x86/hvm/vmx/realmode.c
+++ b/xen/arch/x86/hvm/vmx/realmode.c
@@ -103,7 +103,7 @@ void vmx_realmode_emulate_one(struct hvm_emulate_ctxt 
*hvmemul_ctxt)
 
     rc = hvm_emulate_one(hvmemul_ctxt);
 
-    if ( hvm_vcpu_io_need_completion(vio) || vio->mmio_retry )
+    if ( hvm_vcpu_io_need_completion(vio) )
         vio->io_completion = HVMIO_realmode_completion;
 
     if ( rc == X86EMUL_UNHANDLEABLE )
-- 
2.11.0


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