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Re: [PATCH] target/ppc: fix Hash64 MMU update of PTE bit R

From: Leandro Lupori
Subject: Re: [PATCH] target/ppc: fix Hash64 MMU update of PTE bit R
Date: Wed, 24 Nov 2021 19:17:41 +0000

On 11/24/21 14:40, Daniel Henrique Barboza wrote:
> On 11/24/21 09:00, Leandro Lupori wrote:
>> When updating the R bit of a PTE, the Hash64 MMU was using a wrong byte
>> offset, causing the first byte of the adjacent PTE to be corrupted.
>> This caused a panic when booting FreeBSD, using the Hash MMU.

I wonder how we never hit this issue before. Are you testing PowerNV
and/or pSeries  ?

Could you share a FreeBDS image with us ?
I've hit this issue while testing PowerNV. With pSeries it doesn't happen.

It can be reproduced by trying to boot this iso: https://download.freebsd.org/ftp/snapshots/powerpc/powerpc64/ISO-IMAGES/14.0/FreeBSD-14.0-CURRENT-powerpc-powerpc64-20211028-4827bf76bce-250301-disc1.iso.xz

It is easier to reproduce it using power8/powernv8.

> If you add a "Fixes:" tag with the commit that introduced the code you're
> fixing, we can push it right away as a bug fix in 6.2 (assuming it doesn't
> break anything else, of course).
> The commit to be fixed in the case seems to be a2dd4e83e76b ("ppc/hash64:
> Rework R and C bit updates")


> One more comment below:
>> Signed-off-by: Leandro Lupori <leandro.lupori@eldorado.org.br>
>> ---
>>   target/ppc/mmu-hash64.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>> diff --git a/target/ppc/mmu-hash64.c b/target/ppc/mmu-hash64.c
>> index 19832c4b46..f165ac691a 100644
>> --- a/target/ppc/mmu-hash64.c
>> +++ b/target/ppc/mmu-hash64.c
>> @@ -786,7 +786,7 @@ static void ppc_hash64_set_dsi(CPUState *cs, int mmu_idx, uint64_t dar, uint64_t
>>   static void ppc_hash64_set_r(PowerPCCPU *cpu, hwaddr ptex, uint64_t pte1)
>>   {
>> -    hwaddr base, offset = ptex * HASH_PTE_SIZE_64 + 16;
>> +    hwaddr base, offset = ptex * HASH_PTE_SIZE_64 + 14;
> Instead of adding a '14' you should add a new #define in mmu-hash64.h with this
> value, something like "HPTE64_R_R_SHIFT". This will avoid hardcoding literals
> around the code and forcing us to go to the ISA every time we wonder what's
> an apparently random number represents. There's also a "HPTE64_R_R" defined
> there but I'm not sure if it's usable here, so feel free to create a new
> macro if needed.
> In that note, the original commit that added this code also added a lot of
> hardcoded "15" values for the C bit update in spapr_hpte_set_c() and
> ppc_hash64_set_c(), and a "14" value like you're changing here in spapr_hpte_set_r().
> If you're feeling generous I believe that another patch replacing these hardcoded values
> with bit shift macros is warranted as well.
​What about creating HPTE64_R_R_BYTE and HPTE64_R_C_BYTE, with the values 14 and 15, respectively,
to make it clear that these are byte offsets within a PTE?

May be for 7.0 though ?



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