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Commit 62266869 authored by Avi Kivity's avatar Avi Kivity
Browse files

KVM: x86 emulator: prefetch up to 15 bytes of the instruction executed 



Instead of fetching one byte at a time, prefetch 15 bytes (or until the next
page boundary) to avoid guest page table walks.

Signed-off-by: default avatarAvi Kivity <avi@qumranet.com>
parent 93a0039c

drivers/kvm/x86_emulate.c

+36 −2
Original line number Diff line number Diff line
@@ -414,8 +414,7 @@ static u16 twobyte_table[256] = { 
/* Fetch next part of the instruction being emulated. */

#define insn_fetch(_type, _size, _eip)                                  \

({	unsigned long _x;						\

	rc = ops->read_std((unsigned long)(_eip) + ctxt->cs_base, &_x,	\

			   (_size), ctxt->vcpu);			\

	rc = do_insn_fetch(ctxt, ops, (_eip), &_x, (_size));		\

	if (rc != 0)							\

		goto done;						\

	(_eip) += (_size);						\
@@ -446,6 +445,41 @@ static u16 twobyte_table[256] = { 
		register_address_increment(c->eip, rel);		\

	} while (0)



static int do_fetch_insn_byte(struct x86_emulate_ctxt *ctxt,

			      struct x86_emulate_ops *ops,

			      unsigned long linear, u8 *dest)

{

	struct fetch_cache *fc = &ctxt->decode.fetch;

	int rc;

	int size;



	if (linear < fc->start || linear >= fc->end) {

		size = min(15UL, PAGE_SIZE - offset_in_page(linear));

		rc = ops->read_std(linear, fc->data, size, ctxt->vcpu);

		if (rc)

			return rc;

		fc->start = linear;

		fc->end = linear + size;

	}

	*dest = fc->data[linear - fc->start];

	return 0;

}



static int do_insn_fetch(struct x86_emulate_ctxt *ctxt,

			 struct x86_emulate_ops *ops,

			 unsigned long eip, void *dest, unsigned size)

{

	int rc = 0;



	eip += ctxt->cs_base;

	while (size--) {

		rc = do_fetch_insn_byte(ctxt, ops, eip++, dest++);

		if (rc)

			return rc;

	}

	return 0;

}



/*

 * Given the 'reg' portion of a ModRM byte, and a register block, return a

 * pointer into the block that addresses the relevant register.

drivers/kvm/x86_emulate.h

+7 −0
Original line number Diff line number Diff line
@@ -108,6 +108,12 @@ struct operand { 
	unsigned long val, orig_val, *ptr;

};



struct fetch_cache {

	u8 data[15];

	unsigned long start;

	unsigned long end;

};



struct decode_cache {

	u8 twobyte;

	u8 b;
@@ -130,6 +136,7 @@ struct decode_cache { 
	u8 use_modrm_ea;

	unsigned long modrm_ea;

	unsigned long modrm_val;

	struct fetch_cache fetch;

};



struct x86_emulate_ctxt {