[Xen-devel] [PATCH v3] x86: also allow REP STOS emulation acceleration

["Jan Beulich" <JBeulich@xxxxxxxx>]

While the REP MOVS acceleration appears to have helped qemu-traditional
based guests, qemu-upstream (or really the respective video BIOSes)
doesn't appear to benefit from that. Instead the acceleration added
here provides a visible performance improvement during very early HVM
guest boot.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v3: Drop now pointless memory clobber from asm() in hvmemul_rep_stos()
    Introduce and use ASSERT_UNREACHABLE(), as suggested by Andrew.
v2: Fix asm() constraints in hvmemul_rep_stos(), as pointed out by
    Andrew. Add output operand telling the compiler that "buf" is being
    written.

--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -731,6 +731,17 @@ static int hvmemul_rep_movs_discard(
     return X86EMUL_OKAY;
 }
 
+static int hvmemul_rep_stos_discard(
+    void *p_data,
+    enum x86_segment seg,
+    unsigned long offset,
+    unsigned int bytes_per_rep,
+    unsigned long *reps,
+    struct x86_emulate_ctxt *ctxt)
+{
+    return X86EMUL_OKAY;
+}
+
 static int hvmemul_rep_outs_discard(
     enum x86_segment src_seg,
     unsigned long src_offset,
@@ -982,6 +993,113 @@ static int hvmemul_rep_movs(
     return X86EMUL_OKAY;
 }
 
+static int hvmemul_rep_stos(
+    void *p_data,
+    enum x86_segment seg,
+    unsigned long offset,
+    unsigned int bytes_per_rep,
+    unsigned long *reps,
+    struct x86_emulate_ctxt *ctxt)
+{
+    struct hvm_emulate_ctxt *hvmemul_ctxt =
+        container_of(ctxt, struct hvm_emulate_ctxt, ctxt);
+    unsigned long addr;
+    paddr_t gpa;
+    p2m_type_t p2mt;
+    bool_t df = !!(ctxt->regs->eflags & X86_EFLAGS_DF);
+    int rc = hvmemul_virtual_to_linear(seg, offset, bytes_per_rep, reps,
+                                       hvm_access_write, hvmemul_ctxt, &addr);
+
+    if ( rc == X86EMUL_OKAY )
+    {
+        uint32_t pfec = PFEC_page_present | PFEC_write_access;
+
+        if ( hvmemul_ctxt->seg_reg[x86_seg_ss].attr.fields.dpl == 3 )
+            pfec |= PFEC_user_mode;
+
+        rc = hvmemul_linear_to_phys(
+            addr, &gpa, bytes_per_rep, reps, pfec, hvmemul_ctxt);
+    }
+    if ( rc != X86EMUL_OKAY )
+        return rc;
+
+    /* Check for MMIO op */
+    (void)get_gfn_query_unlocked(current->domain, gpa >> PAGE_SHIFT, &p2mt);
+
+    switch ( p2mt )
+    {
+        unsigned long bytes;
+        void *buf;
+
+    default:
+        /* Allocate temporary buffer. */
+        for ( ; ; )
+        {
+            bytes = *reps * bytes_per_rep;
+            buf = xmalloc_bytes(bytes);
+            if ( buf || *reps <= 1 )
+                break;
+            *reps >>= 1;
+        }
+
+        if ( !buf )
+            buf = p_data;
+        else
+            switch ( bytes_per_rep )
+            {
+                unsigned long dummy;
+
+#define CASE(bits, suffix)                                     \
+            case (bits) / 8:                                   \
+                asm ( "rep stos" #suffix                       \
+                      : "=m" (*(char (*)[bytes])buf),          \
+                        "=D" (dummy), "=c" (dummy)             \
+                      : "a" (*(const uint##bits##_t *)p_data), \
+                         "1" (buf), "2" (*reps) );             \
+                break
+            CASE(8, b);
+            CASE(16, w);
+            CASE(32, l);
+            CASE(64, q);
+#undef CASE
+
+            default:
+                ASSERT_UNREACHABLE();
+                xfree(buf);
+                return X86EMUL_UNHANDLEABLE;
+            }
+
+        /* Adjust address for reverse store. */
+        if ( df )
+            gpa -= bytes - bytes_per_rep;
+
+        rc = hvm_copy_to_guest_phys(gpa, buf, bytes);
+
+        if ( buf != p_data )
+            xfree(buf);
+
+        switch ( rc )
+        {
+        case HVMCOPY_gfn_paged_out:
+        case HVMCOPY_gfn_shared:
+            return X86EMUL_RETRY;
+        case HVMCOPY_okay:
+            return X86EMUL_OKAY;
+        }
+
+        gdprintk(XENLOG_WARNING,
+                 "Failed REP STOS: gpa=%"PRIpaddr" reps=%lu 
bytes_per_rep=%u\n",
+                 gpa, *reps, bytes_per_rep);
+        /* fall through */
+    case p2m_mmio_direct:
+        return X86EMUL_UNHANDLEABLE;
+
+    case p2m_mmio_dm:
+        return hvmemul_do_mmio(gpa, reps, bytes_per_rep, 0, IOREQ_WRITE, df,
+                               p_data);
+    }
+}
+
 static int hvmemul_read_segment(
     enum x86_segment seg,
     struct segment_register *reg,
@@ -1239,6 +1357,7 @@ static const struct x86_emulate_ops hvm_
     .rep_ins       = hvmemul_rep_ins,
     .rep_outs      = hvmemul_rep_outs,
     .rep_movs      = hvmemul_rep_movs,
+    .rep_stos      = hvmemul_rep_stos,
     .read_segment  = hvmemul_read_segment,
     .write_segment = hvmemul_write_segment,
     .read_io       = hvmemul_read_io,
@@ -1264,6 +1383,7 @@ static const struct x86_emulate_ops hvm_
     .rep_ins       = hvmemul_rep_ins_discard,
     .rep_outs      = hvmemul_rep_outs_discard,
     .rep_movs      = hvmemul_rep_movs_discard,
+    .rep_stos      = hvmemul_rep_stos_discard,
     .read_segment  = hvmemul_read_segment,
     .write_segment = hvmemul_write_segment,
     .read_io       = hvmemul_read_io_discard,
--- a/xen/arch/x86/hvm/stdvga.c
+++ b/xen/arch/x86/hvm/stdvga.c
@@ -470,11 +470,11 @@ static int mmio_move(struct hvm_hw_stdvg
     uint64_t addr = p->addr;
     p2m_type_t p2mt;
     struct domain *d = current->domain;
+    int step = p->df ? -p->size : p->size;
 
     if ( p->data_is_ptr )
     {
         uint64_t data = p->data, tmp;
-        int step = p->df ? -p->size : p->size;
 
         if ( p->dir == IOREQ_READ )
         {
@@ -529,13 +529,18 @@ static int mmio_move(struct hvm_hw_stdvg
             }
         }
     }
+    else if ( p->dir == IOREQ_WRITE )
+    {
+        for ( i = 0; i < p->count; i++ )
+        {
+            stdvga_mem_write(addr, p->data, p->size);
+            addr += step;
+        }
+    }
     else
     {
         ASSERT(p->count == 1);
-        if ( p->dir == IOREQ_READ )
-            p->data = stdvga_mem_read(addr, p->size);
-        else
-            stdvga_mem_write(addr, p->data, p->size);
+        p->data = stdvga_mem_read(addr, p->size);
     }
 
     read_data = p->data;
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -2568,15 +2568,25 @@ x86_emulate(
     }
 
     case 0xaa ... 0xab: /* stos */ {
-        /* unsigned long max_reps = */get_rep_prefix();
-        dst.type  = OP_MEM;
+        unsigned long nr_reps = get_rep_prefix();
         dst.bytes = (d & ByteOp) ? 1 : op_bytes;
         dst.mem.seg = x86_seg_es;
         dst.mem.off = truncate_ea(_regs.edi);
-        dst.val   = _regs.eax;
+        if ( (nr_reps == 1) || !ops->rep_stos ||
+             ((rc = ops->rep_stos(&_regs.eax,
+                                  dst.mem.seg, dst.mem.off, dst.bytes,
+                                  &nr_reps, ctxt)) == X86EMUL_UNHANDLEABLE) )
+        {
+            dst.val = _regs.eax;
+            dst.type = OP_MEM;
+            nr_reps = 1;
+        }
+        else if ( rc != X86EMUL_OKAY )
+            goto done;
         register_address_increment(
-            _regs.edi, (_regs.eflags & EFLG_DF) ? -dst.bytes : dst.bytes);
-        put_rep_prefix(1);
+            _regs.edi,
+            nr_reps * ((_regs.eflags & EFLG_DF) ? -dst.bytes : dst.bytes));
+        put_rep_prefix(nr_reps);
         break;
     }
 
--- a/xen/arch/x86/x86_emulate/x86_emulate.h
+++ b/xen/arch/x86/x86_emulate/x86_emulate.h
@@ -241,6 +241,20 @@ struct x86_emulate_ops
         struct x86_emulate_ctxt *ctxt);
 
     /*
+     * rep_stos: Emulate STOS: <*p_data> -> <seg:offset>.
+     *  @bytes_per_rep: [IN ] Bytes transferred per repetition.
+     *  @reps:  [IN ] Maximum repetitions to be emulated.
+     *          [OUT] Number of repetitions actually emulated.
+     */
+    int (*rep_stos)(
+        void *p_data,
+        enum x86_segment seg,
+        unsigned long offset,
+        unsigned int bytes_per_rep,
+        unsigned long *reps,
+        struct x86_emulate_ctxt *ctxt);
+
+    /*
      * read_segment: Emulate a read of full context of a segment register.
      *  @reg:   [OUT] Contents of segment register (visible and hidden state).
      */
--- a/xen/include/xen/lib.h
+++ b/xen/include/xen/lib.h
@@ -41,9 +41,11 @@ do {                                    
 #ifndef NDEBUG
 #define ASSERT(p) \
     do { if ( unlikely(!(p)) ) assert_failed(#p); } while (0)
+#define ASSERT_UNREACHABLE() assert_failed("unreachable")
 #define debug_build() 1
 #else
 #define ASSERT(p) do { if ( 0 && (p) ); } while (0)
+#define ASSERT_UNREACHABLE() do { } while (0)
 #define debug_build() 0
 #endif

Attachment: x86emul-rep-stos.patch
Description: Text document

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