[xen master] x86emul: VCVT{,U}DQ2PD ignores embedded rounding

[patchbot@xxxxxxx]

commit d3709d1324aa140f064b9c68da37547f459f8e8d
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Wed Jan 8 11:01:17 2025 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Jan 8 11:01:17 2025 +0100

    x86emul: VCVT{,U}DQ2PD ignores embedded rounding
    
    IOW we shouldn't raise #UD in that case. Be on the safe side though and
    only encode fully legitimate forms into the stub to be executed.
    
    Things weren't quite right for VCVT{,U}SI2SD either, in the attempt to
    be on the safe side: Clearing EVEX.L'L isn't useful; it's EVEX.b which
    primarily needs clearing. Also reflect the somewhat improved doc
    situation in the comment there.
    
    Fixes: ed806f373730 ("x86emul: support AVX512F legacy-equivalent packed 
int/FP conversion insns")
    Fixes: baf4a376f550 ("x86emul: support AVX512F legacy-equivalent scalar 
int/FP conversion insns")
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/x86_emulate/x86_emulate.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c 
b/xen/arch/x86/x86_emulate/x86_emulate.c
index b89d440133..762fc34778 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -3596,12 +3596,15 @@ x86_emulate(
         if ( !mode_64bit() )
             evex.w = 0;
         /*
-         * SDM version 067 claims that exception type E10NF implies #UD when
-         * EVEX.L'L is non-zero for 32-bit VCVT{,U}SI2SD. Experimentally this
-         * cannot be confirmed, but be on the safe side for the stub.
+         * While SDM version 085 has explicit wording towards embedded rounding
+         * being ignored, it's still not entirely unambiguous with the 
exception
+         * type referred to. Be on the safe side for the stub.
          */
         if ( !evex.w && evex.pfx == vex_f2 )
+        {
+            evex.brs = 0;
             evex.lr = 0;
+        }
         opc[1] = (modrm & 0x38) | 0xc0;
         insn_bytes = EVEX_PFX_BYTES + 2;
         opc[2] = 0xc3;
@@ -4819,7 +4822,16 @@ x86_emulate(
         else
         {
             host_and_vcpu_must_have(avx512f);
-            generate_exception_if(ea.type != OP_MEM && evex.brs, X86_EXC_UD);
+            /*
+             * While SDM version 085 has explicit wording towards embedded
+             * rounding being ignored, it's still not entirely unambiguous with
+             * the exception type referred to. Be on the safe side for the 
stub.
+             */
+            if ( ea.type != OP_MEM && evex.brs )
+            {
+                evex.brs = 0;
+                evex.lr = 2;
+            }
         }
         if ( ea.type != OP_REG || !evex.brs )
             avx512_vlen_check(false);
--
generated by git-patchbot for /home/xen/git/xen.git#master