[Xen-devel] [PATCHv1 3/5] x86/fpu: Add a per-domain field to set the width of FIP/FDP

[David Vrabel <david.vrabel@xxxxxxxxxx>]

The x86 architecture allows either: a) the 64-bit FIP/FDP registers to be
restored (clearing FCS and FDS); or b) the 32-bit FIP/FDP and FCS/FDS
registers to be restored (clearing the upper 32-bits).

Add a per-domain field to indicate which of these options a guest needs.
The options are: 8, 4 or 0.  Where 0 indicates that the hypervisor should
automatically guess the FIP width by checking the value of FIP/FDP when
saving the state (this is the existing behaviour).

The FIP width is initially automatic but is set explicitly in the following
cases:

- 32-bit PV guest: 4
- 64-bit PV guest: 8
- Newer CPUs that do not save FCS/FDS: 8

xsave() has been simplified to not require clearing the FCS/FDS fields --
either these are updated by the XSAVE* instruction or they are not
written.

Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
---
 xen/arch/x86/domain.c        | 10 ++++++++++
 xen/arch/x86/i387.c          | 15 ++++++++-------
 xen/arch/x86/xstate.c        | 43 +++++++++++--------------------------------
 xen/include/asm-x86/domain.h | 15 +++++++++++++++
 4 files changed, 44 insertions(+), 39 deletions(-)

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index 9d43f7b..a15cdf7 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -343,6 +343,8 @@ int switch_native(struct domain *d)
             hvm_set_mode(v, 8);
     }
 
+    d->arch.x87_fip_width = 8;
+
     return 0;
 }
 
@@ -377,6 +379,8 @@ int switch_compat(struct domain *d)
 
     domain_set_alloc_bitsize(d);
 
+    d->arch.x87_fip_width = 4;
+
     return 0;
 
  undo_and_fail:
@@ -653,6 +657,12 @@ int arch_domain_create(struct domain *d, unsigned int 
domcr_flags,
     /* PV/PVH guests get an emulated PIT too for video BIOSes to use. */
     pit_init(d, cpu_khz);
 
+    /*
+     * If the FPU not to save FCS/FDS then we can always save/restore
+     * the 64-bit FIP/FDP and ignore the selectors.
+     */
+    d->arch.x87_fip_width = cpu_has_fpu_sel ? 0 : 8;
+
     return 0;
 
  fail:
diff --git a/xen/arch/x86/i387.c b/xen/arch/x86/i387.c
index 67016c9..ef08a52 100644
--- a/xen/arch/x86/i387.c
+++ b/xen/arch/x86/i387.c
@@ -144,9 +144,9 @@ static inline void fpu_xsave(struct vcpu *v)
 static inline void fpu_fxsave(struct vcpu *v)
 {
     typeof(v->arch.xsave_area->fpu_sse) *fpu_ctxt = v->arch.fpu_ctxt;
-    int word_size = cpu_has_fpu_sel ? 8 : 0;
+    unsigned int fip_width = v->domain->arch.x87_fip_width;
 
-    if ( !is_pv_32bit_vcpu(v) )
+    if ( fip_width != 4 )
     {
         /*
          * The only way to force fxsaveq on a wide range of gas versions.
@@ -164,7 +164,7 @@ static inline void fpu_fxsave(struct vcpu *v)
              boot_cpu_data.x86_vendor == X86_VENDOR_AMD )
             return;
 
-        if ( word_size > 0 &&
+        if ( !fip_width &&
              !((fpu_ctxt->fip.addr | fpu_ctxt->fdp.addr) >> 32) )
         {
             struct ix87_env fpu_env;
@@ -172,17 +172,18 @@ static inline void fpu_fxsave(struct vcpu *v)
             asm volatile ( "fnstenv %0" : "=m" (fpu_env) );
             fpu_ctxt->fip.sel = fpu_env.fcs;
             fpu_ctxt->fdp.sel = fpu_env.fds;
-            word_size = 4;
+            fip_width = 4;
         }
+        else
+            fip_width = 8;
     }
     else
     {
         asm volatile ( "fxsave %0" : "=m" (*fpu_ctxt) );
-        word_size = 4;
+        fip_width = 4;
     }
 
-    if ( word_size >= 0 )
-        fpu_ctxt->x[FPU_WORD_SIZE_OFFSET] = word_size;
+    fpu_ctxt->x[FPU_WORD_SIZE_OFFSET] = fip_width;
 }
 
 /*******************************/
diff --git a/xen/arch/x86/xstate.c b/xen/arch/x86/xstate.c
index 4f2fb8e..c4cc071 100644
--- a/xen/arch/x86/xstate.c
+++ b/xen/arch/x86/xstate.c
@@ -249,7 +249,7 @@ void xsave(struct vcpu *v, uint64_t mask)
     struct xsave_struct *ptr = v->arch.xsave_area;
     uint32_t hmask = mask >> 32;
     uint32_t lmask = mask;
-    int word_size = mask & XSTATE_FP ? (cpu_has_fpu_sel ? 8 : 0) : -1;
+    unsigned int fip_width = v->domain->arch.x87_fip_width;
 #define XSAVE(pfx) \
         alternative_io_3(".byte " pfx "0x0f,0xae,0x27\n", /* xsave */ \
                          ".byte " pfx "0x0f,0xae,0x37\n", /* xsaveopt */ \
@@ -261,28 +261,8 @@ void xsave(struct vcpu *v, uint64_t mask)
                          "=m" (*ptr), \
                          "a" (lmask), "d" (hmask), "D" (ptr))
 
-    if ( word_size <= 0 || !is_pv_32bit_vcpu(v) )
+    if ( fip_width != 4 )
     {
-        typeof(ptr->fpu_sse.fip.sel) fcs = ptr->fpu_sse.fip.sel;
-        typeof(ptr->fpu_sse.fdp.sel) fds = ptr->fpu_sse.fdp.sel;
-
-        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;
-            }
-        }
-
         XSAVE("0x48,");
 
         if ( !(mask & ptr->xsave_hdr.xstate_bv & XSTATE_FP) ||
@@ -293,15 +273,14 @@ void xsave(struct vcpu *v, uint64_t mask)
              (!(ptr->fpu_sse.fsw & 0x0080) &&
               boot_cpu_data.x86_vendor == X86_VENDOR_AMD) )
         {
-            if ( (cpu_has_xsaveopt || cpu_has_xsaves) && word_size > 0 )
-            {
-                ptr->fpu_sse.fip.sel = fcs;
-                ptr->fpu_sse.fdp.sel = fds;
-            }
             return;
         }
 
-        if ( word_size > 0 &&
+        /*
+         * If the FIP/FDP[63:32] are both zero, it is safe to use the
+         * 32-bit restore to also restore the selectors.
+         */
+        if ( !fip_width &&
              !((ptr->fpu_sse.fip.addr | ptr->fpu_sse.fdp.addr) >> 32) )
         {
             struct ix87_env fpu_env;
@@ -309,17 +288,17 @@ void xsave(struct vcpu *v, uint64_t mask)
             asm volatile ( "fnstenv %0" : "=m" (fpu_env) );
             ptr->fpu_sse.fip.sel = fpu_env.fcs;
             ptr->fpu_sse.fdp.sel = fpu_env.fds;
-            word_size = 4;
+            fip_width = 4;
         }
+        else
+            fip_width = 8;
     }
     else
     {
         XSAVE("");
-        word_size = 4;
     }
 #undef XSAVE
-    if ( word_size >= 0 )
-        ptr->fpu_sse.x[FPU_WORD_SIZE_OFFSET] = word_size;
+    ptr->fpu_sse.x[FPU_WORD_SIZE_OFFSET] = fip_width;
 }
 
 void xrstor(struct vcpu *v, uint64_t mask)
diff --git a/xen/include/asm-x86/domain.h b/xen/include/asm-x86/domain.h
index 4fad638..362fd8f 100644
--- a/xen/include/asm-x86/domain.h
+++ b/xen/include/asm-x86/domain.h
@@ -394,6 +394,21 @@ struct arch_domain
 
     /* Emulated devices enabled bitmap. */
     uint32_t emulation_flags;
+
+    /*
+     * The width of the FIP/FDP register in the FPU that needs to be
+     * saved/restored during a context switch.  This is needed because
+     * the FPU can either: a) restore the 64-bit FIP/FDP and clear FCS
+     * and FDS; or b) retstore the 32-bit FIP/FDP (clearing the upper
+     * 32-bits) and restore FCS/FDS.
+     *
+     * Which one is needed depends on the guest.
+     *
+     * This can be either: 8, 4 or 0.  0 means auto-detect the size
+     * based on the width of FIP/FDP values that are written by by the
+     * guest.
+     */
+    uint8_t x87_fip_width;
 } __cacheline_aligned;
 
 #define has_vlapic(d)      (!!((d)->arch.emulation_flags & XEN_X86_EMU_LAPIC))
-- 
2.1.4


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