[PATCH 3/5] x86/xstate: Rework xstate_ctxt_size() as xstate_uncompressed_size()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

We're soon going to need a compressed helper of the same form.

The size of the uncompressed image is a strictly a property of the highest
user state.  This can be calculated trivially with xstate_offsets/sizes, and
is much faster than a CPUID instruction in the first place, let alone the two
XCR0 writes surrounding it.

Retain the cross-check with hardware in debug builds, but forgo it normal
builds.  In particular, this means that the migration paths don't need to mess
with XCR0 just to sanity check the buffer size.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
---
 xen/arch/x86/domctl.c        |  2 +-
 xen/arch/x86/hvm/hvm.c       |  2 +-
 xen/arch/x86/xstate.c        | 40 +++++++++++++++++++++++++++++++---------
 xen/include/asm-x86/xstate.h |  2 +-
 4 files changed, 34 insertions(+), 12 deletions(-)

diff --git a/xen/arch/x86/domctl.c b/xen/arch/x86/domctl.c
index e440bd021e..8c3552410d 100644
--- a/xen/arch/x86/domctl.c
+++ b/xen/arch/x86/domctl.c
@@ -899,7 +899,7 @@ long arch_do_domctl(
         uint32_t offset = 0;
 
 #define PV_XSAVE_HDR_SIZE (2 * sizeof(uint64_t))
-#define PV_XSAVE_SIZE(xcr0) (PV_XSAVE_HDR_SIZE + xstate_ctxt_size(xcr0))
+#define PV_XSAVE_SIZE(xcr0) (PV_XSAVE_HDR_SIZE + 
xstate_uncompressed_size(xcr0))
 
         ret = -ESRCH;
         if ( (evc->vcpu >= d->max_vcpus) ||
diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 28beacc45b..e5fda6b387 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -1203,7 +1203,7 @@ HVM_REGISTER_SAVE_RESTORE(CPU, hvm_save_cpu_ctxt, 
hvm_load_cpu_ctxt, 1,
 
 #define HVM_CPU_XSAVE_SIZE(xcr0) (offsetof(struct hvm_hw_cpu_xsave, \
                                            save_area) + \
-                                  xstate_ctxt_size(xcr0))
+                                  xstate_uncompressed_size(xcr0))
 
 static int hvm_save_cpu_xsave_states(struct vcpu *v, hvm_domain_context_t *h)
 {
diff --git a/xen/arch/x86/xstate.c b/xen/arch/x86/xstate.c
index e6c225a16b..d4c01da574 100644
--- a/xen/arch/x86/xstate.c
+++ b/xen/arch/x86/xstate.c
@@ -184,7 +184,7 @@ void expand_xsave_states(struct vcpu *v, void *dest, 
unsigned int size)
     /* Check there is state to serialise (i.e. at least an XSAVE_HDR) */
     BUG_ON(!v->arch.xcr0_accum);
     /* Check there is the correct room to decompress into. */
-    BUG_ON(size != xstate_ctxt_size(v->arch.xcr0_accum));
+    BUG_ON(size != xstate_uncompressed_size(v->arch.xcr0_accum));
 
     if ( !(xsave->xsave_hdr.xcomp_bv & XSTATE_COMPACTION_ENABLED) )
     {
@@ -246,7 +246,7 @@ void compress_xsave_states(struct vcpu *v, const void *src, 
unsigned int size)
     u64 xstate_bv, valid;
 
     BUG_ON(!v->arch.xcr0_accum);
-    BUG_ON(size != xstate_ctxt_size(v->arch.xcr0_accum));
+    BUG_ON(size != xstate_uncompressed_size(v->arch.xcr0_accum));
     ASSERT(!xsave_area_compressed(src));
 
     xstate_bv = ((const struct xsave_struct *)src)->xsave_hdr.xstate_bv;
@@ -568,16 +568,38 @@ static unsigned int hw_uncompressed_size(uint64_t xcr0)
     return size;
 }
 
-/* Fastpath for common xstate size requests, avoiding reloads of xcr0. */
-unsigned int xstate_ctxt_size(u64 xcr0)
+unsigned int xstate_uncompressed_size(uint64_t xcr0)
 {
-    if ( xcr0 == xfeature_mask )
-        return xsave_cntxt_size;
+    unsigned int size;
+    int idx = flsl(xcr0) - 1;
 
-    if ( xcr0 == 0 )
-        return 0;
+    /*
+     * The maximum size of an uncompressed XSAVE area is determined by the
+     * highest user state, as the size and offset of each component is fixed.
+     */
+    if ( idx >= 2 )
+    {
+        ASSERT(xstate_offsets[idx] && xstate_sizes[idx]);
+        size = xstate_offsets[idx] + xstate_sizes[idx];
+    }
+    else
+        size = XSTATE_AREA_MIN_SIZE;
 
-    return hw_uncompressed_size(xcr0);
+    /* In debug builds, cross-check our calculation with hardware. */
+    if ( IS_ENABLED(CONFIG_DEBUG) )
+    {
+        unsigned int hwsize;
+
+        xcr0 |= XSTATE_FP_SSE;
+        hwsize = hw_uncompressed_size(xcr0);
+
+        if ( size != hwsize )
+            printk_once(XENLOG_ERR "%s(%#"PRIx64") size %#x != hwsize %#x\n",
+                        __func__, xcr0, size, hwsize);
+        size = hwsize;
+    }
+
+    return size;
 }
 
 /* Collect the information of processor's extended state */
diff --git a/xen/include/asm-x86/xstate.h b/xen/include/asm-x86/xstate.h
index 7ab0bdde89..02d6f171b8 100644
--- a/xen/include/asm-x86/xstate.h
+++ b/xen/include/asm-x86/xstate.h
@@ -107,7 +107,7 @@ void compress_xsave_states(struct vcpu *v, const void *src, 
unsigned int size);
 void xstate_free_save_area(struct vcpu *v);
 int xstate_alloc_save_area(struct vcpu *v);
 void xstate_init(struct cpuinfo_x86 *c);
-unsigned int xstate_ctxt_size(u64 xcr0);
+unsigned int xstate_uncompressed_size(uint64_t xcr0);
 
 static inline uint64_t xgetbv(unsigned int index)
 {
-- 
2.11.0