On 11/04/2013 12:44 PM, Alexander Graf wrote:
>
> On 10.10.2013, at 12:16, Paul Mackerras <paulus@xxxxxxxxx> wrote:
>
>> On Wed, Oct 09, 2013 at 10:29:53AM +0200, Alexander Graf wrote:
>>>
>>>
>>> Am 09.10.2013 um 07:59 schrieb Paul Mackerras <paulus@xxxxxxxxx>:
>>>
>>>> On Wed, Oct 09, 2013 at 01:46:29AM +0200, Alexander Graf wrote:
>>>>>
>>>>>
>>>>> Am 09.10.2013 um 01:31 schrieb Paul Mackerras <paulus@xxxxxxxxx>:
>>>>>
>>>>>> True, until we get to POWER8 with its split little-endian support,
>>>>>> where instructions and data can have different endianness...
>>>>>
>>>>> How exactly does that work?
>>>>
>>>> They added an extra MSR bit called SLE which enables the split-endian
>>>> mode. It's bit 5 (IBM numbering). For backwards compatibility, the
>>>> LE bit controls instruction endianness, and data endianness depends on
>>>> LE ^ SLE, that is, with SLE = 0 things work as before. With SLE=1 and
>>>> LE=0 you get little-endian data and big-endian instructions, and vice
>>>> versa with SLE=1 and LE=1.
>>>
>>> So ld32 should only honor LE and get_last_inst only looks at SLE and swaps even the vcpu cached version if it's set, no?
>>
>> Not exactly; instruction endianness depends only on MSR[LE], so
>> get_last_inst should not look at MSR[SLE]. I would think the vcpu
>> cached version should be host endian always.
>
> I agree. It makes the code flow easier.
To take into account the host endian order to determine if we should
byteswap, we could modify kvmppc_need_byteswap() as follow :
+/*
+ * Compare endian order of host and guest to determine whether we need
+ * to byteswap or not
+ */
static inline bool kvmppc_need_byteswap(struct kvm_vcpu *vcpu)
{
- return vcpu->arch.shared->msr & MSR_LE;
+ return ((mfmsr() & (MSR_LE)) >> MSR_LE_LG) ^
+ ((vcpu->arch.shared->msr & (MSR_LE)) >> MSR_LE_LG);
}
and I think MSR[SLE] could be handled this way :
static inline bool kvmppc_is_bigendian(struct kvm_vcpu *vcpu)
@@ -284,10 +289,19 @@ static inline int kvmppc_ld32(struct kvm_vcpu *vcpu, ulong *eaddr,
u32 *ptr, bool data)
{
int ret;
+ bool byteswap;
ret = kvmppc_ld(vcpu, eaddr, sizeof(u32), ptr, data);
- if (kvmppc_need_byteswap(vcpu))
+ byteswap = kvmppc_need_byteswap(vcpu);
+
+ /* if we are loading data from a guest which is in Split
+ * Little Endian mode, the byte order is reversed
+ */
+ if (data && (vcpu->arch.shared->msr & MSR_SLE))
+ byteswap = !byteswap;
+
+ if (byteswap)
*ptr = swab32(*ptr);
return ret;
How does that look ?
This is not tested and the MSR_SLE definition is missing. I will fix that in v5.
Thanks,
C.
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