x86/HVM: suppress I/O completion for port output

We don't break up port requests in case they cross emulation entity
boundaries, and a write to an I/O port is necessarily the last
operation of an instruction instance, so there's no need to re-invoke
the full emulation path upon receiving the result from an external
emulator.

In case we want to properly split port accesses in the future, this
change will need to be reverted, as it would prevent things working
correctly when e.g. the first part needs to go to an external emulator,
while the second part is to be handled internally.

While this addresses the reported problem of Windows paging out the
buffer underneath an in-process REP OUTS, it does not address the wider
problem of the re-issued insn (to the insn emulator) being prone to
raise an exception (#PF) during a replayed, previously successful memory
access (we only record prior MMIO accesses).

Leaving aside the problem tried to be worked around here, I think the
performance aspect alone is a good reason to change the behavior.

Also take the opportunity and change bool_t -> bool as
hvm_vcpu_io_need_completion()'s return type.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>

diff --git a/xen/arch/x86/hvm/emulate.c b/xen/arch/x86/hvm/emulate.c
index 213426db59a236766ee581637f3021a2ac0c119f..f8fca572546d2cfbf36f212a3504e3e7b26469ed 100644 (file)
--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -282,7 +282,11 @@ static int hvmemul_do_io(
             rc = hvm_send_ioreq(s, &p, 0);
             if ( rc != X86EMUL_RETRY || currd->is_shutting_down )
                 vio->io_req.state = STATE_IOREQ_NONE;
-            else if ( data_is_addr )
+            /*
+             * This effectively is !hvm_vcpu_io_need_completion(vio), slightly
+             * optimized and using local variables we have available.
+             */
+            else if ( data_is_addr || (!is_mmio && dir == IOREQ_WRITE) )
                 rc = X86EMUL_OKAY;
         }
         break;

diff --git a/xen/include/asm-x86/hvm/vcpu.h b/xen/include/asm-x86/hvm/vcpu.h
index 2a0403a7f4c90970a048dfd8c93666dd0dbcecf9..fad7b66cca59c19e1ddf661004a7c7aa68000f21 100644 (file)
--- a/xen/include/asm-x86/hvm/vcpu.h
+++ b/xen/include/asm-x86/hvm/vcpu.h
@@ -91,10 +91,12 @@ struct hvm_vcpu_io {
     const struct g2m_ioport *g2m_ioport;
 };
 
-static inline bool_t hvm_vcpu_io_need_completion(const struct hvm_vcpu_io *vio)
+static inline bool hvm_vcpu_io_need_completion(const struct hvm_vcpu_io *vio)
 {
     return (vio->io_req.state == STATE_IOREQ_READY) &&
-           !vio->io_req.data_is_ptr;
+           !vio->io_req.data_is_ptr &&
+           (vio->io_req.type != IOREQ_TYPE_PIO ||
+            vio->io_req.dir != IOREQ_WRITE);
 }
 
 struct nestedvcpu {