[PATCH -v2] kvm: Emulate MOVBE

[Borislav Petkov <bp@alien8.de>]

On Wed, Apr 10, 2013 at 03:16:39PM +0300, Gleb Natapov wrote:
> Right, the question is how kernel can tell QEMU that the cpuid bit is
> supported but should not be set unless explicitly asked by an user.

Actually, this seems to work with the patch below based on whether you
have "+movbe" in the -cpu option or not.

Anyway, here's the second version with hopefully all comments and
suggestions addressed.

Thanks.

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>From 612fc75a732ad16332f270b7c52a68c89e3565ca Mon Sep 17 00:00:00 2001
From: Borislav Petkov <bp@suse.de>
Date: Thu, 11 Apr 2013 02:06:30 +0200
Subject: [PATCH] kvm: Emulate MOVBE

This basically came from the need to be able to boot 32-bit Atom SMP
guests on an AMD host, i.e. host which doesn't support MOVBE. As a
matter of fact, qemu has since recently received MOVBE support but we
cannot share that with kvm emulation and thus we have to do this in the
host.

We piggyback on the #UD path and emulate the MOVBE functionality. With
it, an 8-core SMP guest boots in under 6 seconds.

Also, requesting MOVBE emulation needs to happen explicitly to work,
i.e. qemu -cpu n270,+movbe...

Signed-off-by: Andre Przywara <andre@andrep.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
---
 arch/x86/kvm/cpuid.c   |  2 +-
 arch/x86/kvm/emulate.c | 39 +++++++++++++++++++++++++++++++++++++--
 2 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index a20ecb5b6cbf..2d44fc4fd855 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -273,7 +273,7 @@ static int do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
 		cpuid_mask(&entry->ecx, 4);
 		/* we support x2apic emulation even if host does not support
 		 * it since we emulate x2apic in software */
-		entry->ecx |= F(X2APIC);
+		entry->ecx |= F(X2APIC) | F(MOVBE);
 		break;
 	/* function 2 entries are STATEFUL. That is, repeated cpuid commands
 	 * may return different values. This forces us to get_cpu() before
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index a335cc6cde72..9011c7a656ad 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -152,6 +152,7 @@
 #define Avx         ((u64)1 << 43)  /* Advanced Vector Extensions */
 #define Fastop      ((u64)1 << 44)  /* Use opcode::u.fastop */
 #define NoWrite     ((u64)1 << 45)  /* No writeback */
+#define EmulateOnUD ((u64)1 << 46)  /* emulate if unsupported by the host */
 
 #define X2(x...) x, x
 #define X3(x...) X2(x), x
@@ -3107,6 +3108,30 @@ static int em_mov(struct x86_emulate_ctxt *ctxt)
 	return X86EMUL_CONTINUE;
 }
 
+static int em_movbe(struct x86_emulate_ctxt *ctxt)
+{
+	switch (ctxt->op_bytes) {
+	case 2:
+		*(u16 *)ctxt->dst.valptr = swab16(*(u16 *)ctxt->src.valptr);
+		break;
+	case 4:
+		*(u32 *)ctxt->dst.valptr = swab32(*(u32 *)ctxt->src.valptr);
+
+		/*
+		 * clear upper dword for 32-bit operand size in 64-bit mode.
+		 */
+		if (ctxt->mode == X86EMUL_MODE_PROT64)
+			*((u32 *)ctxt->dst.valptr + 1) = 0x0;
+		break;
+	case 8:
+		*(u64 *)ctxt->dst.valptr = swab64(*(u64 *)ctxt->src.valptr);
+		break;
+	default:
+		return X86EMUL_PROPAGATE_FAULT;
+	}
+	return X86EMUL_CONTINUE;
+}
+
 static int em_cr_write(struct x86_emulate_ctxt *ctxt)
 {
 	if (ctxt->ops->set_cr(ctxt, ctxt->modrm_reg, ctxt->src.val))
@@ -4033,6 +4058,11 @@ static const struct opcode twobyte_table[256] = {
 	N, N, N, N, N, N, N, N, N, N, N, N, N, N, N, N
 };
 
+static const struct opcode threebyte_table[] = {
+	[0xf0] = I(DstReg | SrcMem | ModRM | Mov | EmulateOnUD, em_movbe),
+	[0xf1] = I(DstMem | SrcReg | ModRM | Mov | EmulateOnUD, em_movbe),
+};
+
 #undef D
 #undef N
 #undef G
@@ -4320,6 +4350,9 @@ done_prefixes:
 		ctxt->twobyte = 1;
 		ctxt->b = insn_fetch(u8, ctxt);
 		opcode = twobyte_table[ctxt->b];
+
+		if (ctxt->b == 0x38)
+			opcode = threebyte_table[insn_fetch(u8, ctxt)];
 	}
 	ctxt->d = opcode.flags;
 
@@ -4376,8 +4409,10 @@ done_prefixes:
 	if (ctxt->d == 0 || (ctxt->d & Undefined))
 		return EMULATION_FAILED;
 
-	if (!(ctxt->d & VendorSpecific) && ctxt->only_vendor_specific_insn)
-		return EMULATION_FAILED;
+	if (!(ctxt->d & VendorSpecific) && ctxt->only_vendor_specific_insn) {
+		if (!(ctxt->d & EmulateOnUD))
+			return EMULATION_FAILED;
+	}
 
 	if (mode == X86EMUL_MODE_PROT64 && (ctxt->d & Stack))
 		ctxt->op_bytes = 8;
-- 
1.8.2.135.g7b592fa


-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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