[Xen-devel] [PATCH] x86emul: adjust handling of AVX2 gathers

["Jan Beulich" <JBeulich@xxxxxxxx>]

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@xxxxxxxx>

--- 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 )




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