Re: [Qemu-devel] KVM-PR is broken with current QEMU

[Thomas Huth]

On 20.09.2016 23:45, Benjamin Herrenschmidt wrote:
> On Tue, 2016-09-20 at 13:44 +0200, Thomas Huth wrote:
>>
>> Seems like KVM PR is using the "degraded" ISA variants (without the
>> 1TB
>> segments), but the new POWERPC_MMU_64K flag has not been added to
>> those.
>> Has this been done on purpose, or was this just by accident?
>> I can make KVM PR working again with the following patch:
>>
>> diff --git a/target-ppc/cpu-qom.h b/target-ppc/cpu-qom.h
>> index 2864105..36694cb 100644
>> --- a/target-ppc/cpu-qom.h
>> +++ b/target-ppc/cpu-qom.h
>> @@ -81,6 +81,7 @@ enum powerpc_mmu_t {
>>                               | POWERPC_MMU_AMR | 0x00000003,
>>      /* Architecture 2.06 "degraded" (no 1T segments)           */
>>      POWERPC_MMU_2_06a      = POWERPC_MMU_64 | POWERPC_MMU_AMR
>> +                             | POWERPC_MMU_64K
>>                               | 0x00000003,
>>      /* Architecture 2.07 variant                               */
>>      POWERPC_MMU_2_07       = POWERPC_MMU_64 | POWERPC_MMU_1TSEG
>> @@ -88,6 +89,7 @@ enum powerpc_mmu_t {
>>                               | POWERPC_MMU_AMR | 0x00000004,
>>      /* Architecture 2.07 "degraded" (no 1T segments)           */
>>      POWERPC_MMU_2_07a      = POWERPC_MMU_64 | POWERPC_MMU_AMR
>> +                             | POWERPC_MMU_64K
>>                               | 0x00000004,
>>  };
>>
>> However, not sure whether this is the right fix ... Cédric, Ben, any
>> ideas?
> 
> Oh I thought I had removed the degraded variants ... Definitely looks
> like an accident. I *think* PR KVM supports 64K pages, no ? If not,
> then we shouldn't enable the flag.. somebody needs to check the kernel.

I've now added some debug printf statements to kvm_fixup_page_sizes() in
QEMU, and it seems that at least my current downstream kernel does not
report support for 64K page sizes. So the right fix is likely to disable
the POWERPC_MMU_64K bit there in env->mmu_model if the kernel does not
report support for 64K pages. I'll try to come up with a patch...

But actually, now I wonder why my kernel does not support this page
size. There was a kernel patch from Paul (a4a0f2524acc2c6 - "KVM: PPC:
Book3S PR: Allow guest to use 64k pages") which added this page size to
KVM-PR, so IMHO it should work ... Seem like I need to do some more
debugging here...

 Thomas