Commit 9d643f63 authored by Nick Desaulniers's avatar Nick Desaulniers Committed by Paolo Bonzini
Browse files

KVM: x86: avoid large stack allocations in em_fxrstor 



em_fxstor previously called fxstor_fixup.  Both created instances of
struct fxregs_state on the stack, which triggered the warning:

arch/x86/kvm/emulate.c:4018:12: warning: stack frame size of 1080 bytes
in function
      'em_fxrstor' [-Wframe-larger-than=]
static int em_fxrstor(struct x86_emulate_ctxt *ctxt)
           ^
with CONFIG_FRAME_WARN set to 1024.

This patch does the fixup in em_fxstor now, avoiding one additional
struct fxregs_state, and now fxstor_fixup can be removed as it has no
other call sites.

Further, the calculation for offsets into xmm_space can be shared
between em_fxstor and em_fxsave.

Signed-off-by: default avatarNick Desaulniers <nick.desaulniers@gmail.com>
[Clean up calculation of offsets and fix it for 64-bit mode. - Paolo]
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 7461fbc4

arch/x86/kvm/emulate.c

+37 −45
Original line number Diff line number Diff line
@@ -3940,6 +3940,25 @@ static int check_fxsr(struct x86_emulate_ctxt *ctxt) 
	return X86EMUL_CONTINUE;

}



/*

 * Hardware doesn't save and restore XMM 0-7 without CR4.OSFXSR, but does save

 * and restore MXCSR.

 */

static size_t __fxstate_size(int nregs)

{

	return offsetof(struct fxregs_state, xmm_space[0]) + nregs * 16;

}



static inline size_t fxstate_size(struct x86_emulate_ctxt *ctxt)

{

	bool cr4_osfxsr;

	if (ctxt->mode == X86EMUL_MODE_PROT64)

		return __fxstate_size(16);



	cr4_osfxsr = ctxt->ops->get_cr(ctxt, 4) & X86_CR4_OSFXSR;

	return __fxstate_size(cr4_osfxsr ? 8 : 0);

}



/*

 * FXSAVE and FXRSTOR have 4 different formats depending on execution mode,

 *  1) 16 bit mode
@@ -3961,7 +3980,6 @@ static int check_fxsr(struct x86_emulate_ctxt *ctxt) 
static int em_fxsave(struct x86_emulate_ctxt *ctxt)

{

	struct fxregs_state fx_state;

	size_t size;

	int rc;



	rc = check_fxsr(ctxt);
@@ -3977,68 +3995,42 @@ static int em_fxsave(struct x86_emulate_ctxt *ctxt) 
	if (rc != X86EMUL_CONTINUE)

		return rc;



	if (ctxt->ops->get_cr(ctxt, 4) & X86_CR4_OSFXSR)

		size = offsetof(struct fxregs_state, xmm_space[8 * 16/4]);

	else

		size = offsetof(struct fxregs_state, xmm_space[0]);



	return segmented_write_std(ctxt, ctxt->memop.addr.mem, &fx_state, size);

}



static int fxrstor_fixup(struct x86_emulate_ctxt *ctxt,

		struct fxregs_state *new)

{

	int rc = X86EMUL_CONTINUE;

	struct fxregs_state old;



	rc = asm_safe("fxsave %[fx]", , [fx] "+m"(old));

	if (rc != X86EMUL_CONTINUE)

		return rc;



	/*

	 * 64 bit host will restore XMM 8-15, which is not correct on non-64

	 * bit guests.  Load the current values in order to preserve 64 bit

	 * XMMs after fxrstor.

	 */

#ifdef CONFIG_X86_64

	/* XXX: accessing XMM 8-15 very awkwardly */

	memcpy(&new->xmm_space[8 * 16/4], &old.xmm_space[8 * 16/4], 8 * 16);

#endif



	/*

	 * Hardware doesn't save and restore XMM 0-7 without CR4.OSFXSR, but

	 * does save and restore MXCSR.

	 */

	if (!(ctxt->ops->get_cr(ctxt, 4) & X86_CR4_OSFXSR))

		memcpy(new->xmm_space, old.xmm_space, 8 * 16);



	return rc;

	return segmented_write_std(ctxt, ctxt->memop.addr.mem, &fx_state,

		                   fxstate_size(ctxt));

}



static int em_fxrstor(struct x86_emulate_ctxt *ctxt)

{

	struct fxregs_state fx_state;

	int rc;

	size_t size;



	rc = check_fxsr(ctxt);

	if (rc != X86EMUL_CONTINUE)

		return rc;



	rc = segmented_read_std(ctxt, ctxt->memop.addr.mem, &fx_state, 512);

	if (rc != X86EMUL_CONTINUE)

		return rc;

	ctxt->ops->get_fpu(ctxt);



	if (fx_state.mxcsr >> 16)

		return emulate_gp(ctxt, 0);

	size = fxstate_size(ctxt);

	if (size < __fxstate_size(16)) {

		rc = asm_safe("fxsave %[fx]", , [fx] "+m"(fx_state));

		if (rc != X86EMUL_CONTINUE)

			goto out;

	}



	ctxt->ops->get_fpu(ctxt);

	rc = segmented_read_std(ctxt, ctxt->memop.addr.mem, &fx_state, size);

	if (rc != X86EMUL_CONTINUE)

		goto out;



	if (ctxt->mode < X86EMUL_MODE_PROT64)

		rc = fxrstor_fixup(ctxt, &fx_state);

	if (fx_state.mxcsr >> 16) {

		rc = emulate_gp(ctxt, 0);

		goto out;

	}



	if (rc == X86EMUL_CONTINUE)

		rc = asm_safe("fxrstor %[fx]", : [fx] "m"(fx_state));



out:

	ctxt->ops->put_fpu(ctxt);



	return rc;

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