[Xen-devel] [PATCHv2 1/3] x86/fpu: improve check for XSAVE* not writing FIP/FDP fields

[David Vrabel <david.vrabel@xxxxxxxxxx>]

The hardware may not write the FIP/FDP fields with a XSAVE*
instruction.  e.g., with XSAVEOPT/XSAVES if the state hasn't changed
or on AMD CPUs when a floating point exception is not pending.  We
need to identify this case so we can correctly apply the check for
whether to save/restore FCS/FDS.

By toggling FIP[63] we can turn the field into a non-canonical address
and check for this value after the XSAVE instruction.

This results in smaller code with fewer branches and is more
understandable.

Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
---
 xen/arch/x86/xstate.c | 43 +++++++++++++------------------------------
 1 file changed, 13 insertions(+), 30 deletions(-)

diff --git a/xen/arch/x86/xstate.c b/xen/arch/x86/xstate.c
index 4f2fb8e..0869789 100644
--- a/xen/arch/x86/xstate.c
+++ b/xen/arch/x86/xstate.c
@@ -263,41 +263,24 @@ void xsave(struct vcpu *v, uint64_t mask)
 
     if ( word_size <= 0 || !is_pv_32bit_vcpu(v) )
     {
-        typeof(ptr->fpu_sse.fip.sel) fcs = ptr->fpu_sse.fip.sel;
-        typeof(ptr->fpu_sse.fdp.sel) fds = ptr->fpu_sse.fdp.sel;
+        uint64_t bad_fip;
 
-        if ( cpu_has_xsaveopt || cpu_has_xsaves )
-        {
-            /*
-             * XSAVEOPT/XSAVES may not write the FPU portion even when the
-             * respective mask bit is set. For the check further down to work
-             * we hence need to put the save image back into the state that
-             * it was in right after the previous XSAVEOPT.
-             */
-            if ( word_size > 0 &&
-                 (ptr->fpu_sse.x[FPU_WORD_SIZE_OFFSET] == 4 ||
-                  ptr->fpu_sse.x[FPU_WORD_SIZE_OFFSET] == 2) )
-            {
-                ptr->fpu_sse.fip.sel = 0;
-                ptr->fpu_sse.fdp.sel = 0;
-            }
-        }
+        /*
+         * FIP/FDP may not be written in some cases (e.g., if
+         * XSAVEOPT/XSAVES is used, or on AMD CPUs if an exception
+         * isn't pending).
+         *
+         * To tell if the hardware writes these fields, make the FIP
+         * field non-canonical by flipping the top bit.
+         */
+        bad_fip = ptr->fpu_sse.fip.addr ^= 1ull << 63;
 
         XSAVE("0x48,");
 
-        if ( !(mask & ptr->xsave_hdr.xstate_bv & XSTATE_FP) ||
-             /*
-              * AMD CPUs don't save/restore FDP/FIP/FOP unless an exception
-              * is pending.
-              */
-             (!(ptr->fpu_sse.fsw & 0x0080) &&
-              boot_cpu_data.x86_vendor == X86_VENDOR_AMD) )
+        /* FIP/FDP not updated? Restore the old FIP value. */
+        if ( ptr->fpu_sse.fip.addr == bad_fip )
         {
-            if ( (cpu_has_xsaveopt || cpu_has_xsaves) && word_size > 0 )
-            {
-                ptr->fpu_sse.fip.sel = fcs;
-                ptr->fpu_sse.fdp.sel = fds;
-            }
+            ptr->fpu_sse.fip.addr ^= 1ull << 63;
             return;
         }
 
-- 
2.1.4


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