x86emul: adjust handling of AVX2 gathers

HVM's MMIO cache only has a capacity of three entries. Once running out
of entries, hvmemul_linear_mmio_access() will return
X86EMUL_UNHANDLEABLE. Since gathers are an iterative process anyway,
simply commit the portion of work done in this and hypothetical similar
cases, exiting back to guest context for the insn to be retried.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>

diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c b/xen/arch/x86/x86_emulate/x86_emulate.c
index 3389050ab459dd8289c23be7a8c4d473e2c0bf21..8ffda8384a6899e388c782bec198f00bea5358bd 100644 (file)
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -7639,6 +7639,7 @@ x86_emulate(
             int32_t dw[8];
             int64_t qw[4];
         } index, mask;
+        bool done = false;
 
         ASSERT(ea.type == OP_MEM);
         generate_exception_if(modrm_reg == state->sib_index ||
@@ -7692,12 +7693,23 @@ x86_emulate(
                                ea.mem.off + (idx << state->sib_scale),
                                (void *)mmvalp + i * op_bytes, op_bytes, ctxt);
                 if ( rc != X86EMUL_OKAY )
+                {
+                    /*
+                     * If we've made any progress and the access did not fault,
+                     * force a retry instead. This is for example necessary to
+                     * cope with the limited capacity of HVM's MMIO cache.
+                     */
+                    if ( rc != X86EMUL_EXCEPTION && done )
+                        rc = X86EMUL_RETRY;
                     break;
+                }
 
 #ifdef __XEN__
                 if ( i + 1 < n && local_events_need_delivery() )
                     rc = X86EMUL_RETRY;
 #endif
+
+                done = true;
             }
 
             if ( vex.w )