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

[Daniel Henrique Barboza <danielhb413@gmail.com>]

On 11/24/21 16:17, Leandro Lupori wrote:
> ​​
> 
> 
>     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")
> 
>     Indeed.
> 
> ​​Right.
> 
>     > 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_BYTEand HPTE64_R_C_BYTE, with the values 14 and 15, respectively,
> to make it clear that these are byte offsets within a PTE?

Looks good to me.


Daniel


> 
>     May be for 7.0 though ?
> 
>     Thanks,
> 
>     C.
>