Re: [PATCH 3/3] KVM: PPC: Book3S: MMIO emulation support for little endian guests

[Paul Mackerras <paulus@samba.org>]

On Mon, Oct 07, 2013 at 04:27:47PM +0200, Cédric Le Goater wrote:
> MMIO emulation reads the last instruction executed by the guest
> and then emulates. If the guest is running in Little Endian mode,
> the instruction needs to be byte-swapped before being emulated.
> 
> This patch stores the last instruction in the endian order of the
> host, primarily doing a byte-swap if needed. The common code
> which fetches 'last_inst' uses a helper routine kvmppc_need_byteswap().
> and the exit paths for the Book3S PV and HR guests use their own
> version in assembly.
> 
> Finally, kvmppc_emulate_instruction() uses kvmppc_is_bigendian()
> to define in which endian order the mmio needs to be done.
> 
> Signed-off-by: Cédric Le Goater <clg@fr.ibm.com>

[snip]

> +	ld	r0, VCPU_MSR(r9)
> +
> +	/* r10 = vcpu->arch.msr & MSR_LE */
> +	rldicl.	r10, r0, 0, 63

I would have written:

	andi.	r10, r0, MSR_LE

which doesn't need the comment, but really the two are equivalent.

> @@ -232,6 +231,7 @@ int kvmppc_emulate_instruction(struct kvm_run *run, struct kvm_vcpu *vcpu)
>  	int sprn = get_sprn(inst);
>  	enum emulation_result emulated = EMULATE_DONE;
>  	int advance = 1;
> +	int is_bigendian = kvmppc_is_bigendian(vcpu);
>  
>  	/* this default type might be overwritten by subcategories */
>  	kvmppc_set_exit_type(vcpu, EMULATED_INST_EXITS);
> @@ -266,47 +266,53 @@ int kvmppc_emulate_instruction(struct kvm_run *run, struct kvm_vcpu *vcpu)
>  			advance = 0;
>  			break;
>  		case OP_31_XOP_LWZX:
> -			emulated = kvmppc_handle_load(run, vcpu, rt, 4, 1);
> +			emulated = kvmppc_handle_load(run, vcpu, rt, 4,
> +						      is_bigendian);

I see you're still hitting all the call sites of kvmppc_handle_load(),
kvmppc_handle_loads() and kvmppc_handle_store(), rather than putting
the big-endian test inside kvmppc_handle_load() and
kvmppc_handle_store(), as in this untested patch:

diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
index f55e14c..171bce6 100644
--- a/arch/powerpc/kvm/powerpc.c
+++ b/arch/powerpc/kvm/powerpc.c
@@ -625,9 +625,13 @@ static void kvmppc_complete_mmio_load(struct kvm_vcpu *vcpu,
 }
 
 int kvmppc_handle_load(struct kvm_run *run, struct kvm_vcpu *vcpu,
-                       unsigned int rt, unsigned int bytes, int is_bigendian)
+                       unsigned int rt, unsigned int bytes, int not_reverse)
 {
 	int idx, ret;
+	int is_bigendian = not_reverse;
+
+	if (!kvmppc_is_bigendian(vcpu))
+		is_bigendian = !not_reverse;
 
 	if (bytes > sizeof(run->mmio.data)) {
 		printk(KERN_ERR "%s: bad MMIO length: %d\n", __func__,
@@ -662,21 +666,25 @@ int kvmppc_handle_load(struct kvm_run *run, struct kvm_vcpu *vcpu,
 
 /* Same as above, but sign extends */
 int kvmppc_handle_loads(struct kvm_run *run, struct kvm_vcpu *vcpu,
-                        unsigned int rt, unsigned int bytes, int is_bigendian)
+                        unsigned int rt, unsigned int bytes, int not_reverse)
 {
 	int r;
 
 	vcpu->arch.mmio_sign_extend = 1;
-	r = kvmppc_handle_load(run, vcpu, rt, bytes, is_bigendian);
+	r = kvmppc_handle_load(run, vcpu, rt, bytes, not_reverse);
 
 	return r;
 }
 
 int kvmppc_handle_store(struct kvm_run *run, struct kvm_vcpu *vcpu,
-                        u64 val, unsigned int bytes, int is_bigendian)
+                        u64 val, unsigned int bytes, int not_reverse)
 {
 	void *data = run->mmio.data;
 	int idx, ret;
+	int is_bigendian = not_reverse;
+
+	if (!kvmppc_is_bigendian(vcpu))
+		is_bigendian = !not_reverse;
 
 	if (bytes > sizeof(run->mmio.data)) {
 		printk(KERN_ERR "%s: bad MMIO length: %d\n", __func__,

That seems simpler to me -- is there a reason not to do it that way?

Paul.