[PATCH v2 2/2] x86/fpu: Split fpu_setup_fpu() in two

[Alejandro Vallejo <alejandro.vallejo@cloud.com>]

It was trying to do too many things at once and there was no clear way of
defining what it was meant to do.This commit splits the function in two.

  1. A reset function, parameterized by the FCW value. FCW_RESET means to reset
     the state to power-on reset values, while FCW_DEFAULT means to reset to the
     default values present during vCPU creation.
  2. A x87/SSE state loader (equivalent to the old function when it took a data
     pointer).

While at it, make sure the abridged tag is consistent with the manuals and
start as 0xFF.

Signed-off-by: Alejandro Vallejo <alejandro.vallejo@cloud.com>
---
v2:

  * Reworded comment about pre-Xen 4.1 migrations.
  * Reset abridged FTW to -1 (tag=0x5555), as per the manuals.
  * Undo const cast-away.
  * Split vcpu_reset_fpu() into vcpu_default_fpu()
    * vcpu_init_fpu() already exists.
  * Removed backticks from comments
---
 xen/arch/x86/domain.c             |  7 ++--
 xen/arch/x86/hvm/hvm.c            | 27 ++++++++++----
 xen/arch/x86/i387.c               | 60 +++++++++++++++----------------
 xen/arch/x86/include/asm/i387.h   | 28 ++++++++++++---
 xen/arch/x86/include/asm/xstate.h |  1 +
 5 files changed, 77 insertions(+), 46 deletions(-)

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index d977ec71ca20..5af9e3e7a8b4 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -1186,9 +1186,10 @@ int arch_set_info_guest(
          is_pv_64bit_domain(d) )
         v->arch.flags &= ~TF_kernel_mode;
 
-    vcpu_setup_fpu(v, v->arch.xsave_area,
-                   flags & VGCF_I387_VALID ? &c.nat->fpu_ctxt : NULL,
-                   FCW_DEFAULT);
+    if ( flags & VGCF_I387_VALID )
+        vcpu_setup_fpu(v, &c.nat->fpu_ctxt);
+    else
+        vcpu_default_fpu(v);
 
     if ( !compat )
     {
diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 6607dba562a4..83cb21884ce6 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -1164,10 +1164,25 @@ static int cf_check hvm_load_cpu_ctxt(struct domain *d, hvm_domain_context_t *h)
     seg.attr = ctxt.ldtr_arbytes;
     hvm_set_segment_register(v, x86_seg_ldtr, &seg);
 
-    /* Cover xsave-absent save file restoration on xsave-capable host. */
-    vcpu_setup_fpu(v, xsave_enabled(v) ? NULL : v->arch.xsave_area,
-                   ctxt.flags & XEN_X86_FPU_INITIALISED ? ctxt.fpu_regs : NULL,
-                   FCW_RESET);
+    /*
+     * On Xen 4.1 and later the FPU state is restored on later HVM context in
+     * the migrate stream, so what we're doing here is initialising the FPU
+     * state for guests from even older versions of Xen.
+     *
+     * In particular:
+     *   1. If there's an XSAVE context later in the stream what we do here for
+     *      the FPU doesn't matter because it'll be overriden later.
+     *   2. If there isn't and the guest didn't use extended states it's still
+     *      fine because we have all the information we need here.
+     *   3. If there isn't and the guest DID use extended states (could've
+     *      happened prior to Xen 4.1) then we're in a pickle because we have
+     *      to make up non-existing state. For this case we initialise the FPU
+     *      as using x87/SSE only because the rest of the state is gone.
+     */
+    if ( ctxt.flags & XEN_X86_FPU_INITIALISED )
+        vcpu_setup_fpu(v, &ctxt.fpu_regs);
+    else
+        vcpu_reset_fpu(v);
 
     v->arch.user_regs.rax = ctxt.rax;
     v->arch.user_regs.rbx = ctxt.rbx;
@@ -4007,9 +4022,7 @@ void hvm_vcpu_reset_state(struct vcpu *v, uint16_t cs, uint16_t ip)
         v->arch.guest_table = pagetable_null();
     }
 
-    if ( v->arch.xsave_area )
-        v->arch.xsave_area->xsave_hdr.xstate_bv = 0;
-    vcpu_setup_fpu(v, v->arch.xsave_area, NULL, FCW_RESET);
+    vcpu_reset_fpu(v);
 
     arch_vcpu_regs_init(v);
     v->arch.user_regs.rip = ip;
diff --git a/xen/arch/x86/i387.c b/xen/arch/x86/i387.c
index fbb9d3584a3d..af5ae805998a 100644
--- a/xen/arch/x86/i387.c
+++ b/xen/arch/x86/i387.c
@@ -303,41 +303,37 @@ int vcpu_init_fpu(struct vcpu *v)
     return xstate_alloc_save_area(v);
 }
 
-void vcpu_setup_fpu(struct vcpu *v, struct xsave_struct *xsave_area,
-                    const void *data, unsigned int fcw_default)
+void vcpu_reset_fpu(struct vcpu *v)
 {
-    fpusse_t *fpu_sse = &v->arch.xsave_area->fpu_sse;
-
-    ASSERT(!xsave_area || xsave_area == v->arch.xsave_area);
-
-    v->fpu_initialised = !!data;
+    v->fpu_initialised = false;
+    *v->arch.xsave_area = (struct xsave_struct) {
+        .fpu_sse = {
+            .mxcsr = MXCSR_DEFAULT,
+            .fcw = FCW_RESET,
+            .ftw = FTW_RESET,
+        },
+        .xsave_hdr.xstate_bv = fcw == X86_XCR0_X87,
+    };
+}
 
-    if ( data )
-    {
-        memcpy(fpu_sse, data, sizeof(*fpu_sse));
-        if ( xsave_area )
-            xsave_area->xsave_hdr.xstate_bv = XSTATE_FP_SSE;
-    }
-    else if ( xsave_area && fcw_default == FCW_DEFAULT )
-    {
-        xsave_area->xsave_hdr.xstate_bv = 0;
-        fpu_sse->mxcsr = MXCSR_DEFAULT;
-    }
-    else
-    {
-        memset(fpu_sse, 0, sizeof(*fpu_sse));
-        fpu_sse->fcw = fcw_default;
-        fpu_sse->mxcsr = MXCSR_DEFAULT;
-        if ( v->arch.xsave_area )
-        {
-            v->arch.xsave_area->xsave_hdr.xstate_bv &= ~XSTATE_FP_SSE;
-            if ( fcw_default != FCW_DEFAULT )
-                v->arch.xsave_area->xsave_hdr.xstate_bv |= X86_XCR0_X87;
-        }
-    }
+void vcpu_default_fpu(struct vcpu *v)
+{
+    v->fpu_initialised = false;
+    *v->arch.xsave_area = (struct xsave_struct) {
+        .fpu_sse = {
+            .mxcsr = MXCSR_DEFAULT,
+            .fcw = FCW_DEFAULT,
+        },
+    };
+}
 
-    if ( xsave_area )
-        xsave_area->xsave_hdr.xcomp_bv = 0;
+void vcpu_setup_fpu(struct vcpu *v, const void *data)
+{
+    v->fpu_initialised = true;
+    *v->arch.xsave_area = (struct xsave_struct) {
+        .fpu_sse = *(const fpusse_t*)data,
+        .xsave_hdr.xstate_bv = XSTATE_FP_SSE,
+    };
 }
 
 /* Free FPU's context save area */
diff --git a/xen/arch/x86/include/asm/i387.h b/xen/arch/x86/include/asm/i387.h
index a783549db991..7a69577de45b 100644
--- a/xen/arch/x86/include/asm/i387.h
+++ b/xen/arch/x86/include/asm/i387.h
@@ -31,10 +31,30 @@ void vcpu_restore_fpu_nonlazy(struct vcpu *v, bool need_stts);
 void vcpu_restore_fpu_lazy(struct vcpu *v);
 void vcpu_save_fpu(struct vcpu *v);
 void save_fpu_enable(void);
-
 int vcpu_init_fpu(struct vcpu *v);
-struct xsave_struct;
-void vcpu_setup_fpu(struct vcpu *v, struct xsave_struct *xsave_area,
-                    const void *data, unsigned int fcw_default);
 void vcpu_destroy_fpu(struct vcpu *v);
+
+/*
+ * Restore v's FPU to power-on reset values
+ *
+ * @param v vCPU containing the FPU
+ */
+void vcpu_reset_fpu(struct vcpu *v);
+
+/*
+ * Restore v's FPU to default values
+ *
+ * @param v vCPU containing the FPU
+ */
+void vcpu_default_fpu(struct vcpu *v);
+
+/*
+ * Load x87/SSE state into v's FPU
+ *
+ * Overrides the XSAVE header to set the state components to be x87 and SSE.
+ *
+ * @param v    vCPU containing the FPU
+ * @param data 512-octet blob for x87/SSE state
+ */
+void vcpu_setup_fpu(struct vcpu *v, const void *data);
 #endif /* __ASM_I386_I387_H */
diff --git a/xen/arch/x86/include/asm/xstate.h b/xen/arch/x86/include/asm/xstate.h
index ebeb2a3dcaf9..6144ed6f8551 100644
--- a/xen/arch/x86/include/asm/xstate.h
+++ b/xen/arch/x86/include/asm/xstate.h
@@ -14,6 +14,7 @@
 
 #define FCW_DEFAULT               0x037f
 #define FCW_RESET                 0x0040
+#define FTW_RESET                 0xFF
 #define MXCSR_DEFAULT             0x1f80
 
 extern uint32_t mxcsr_mask;
-- 
2.45.2