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Subject: Re: [PATCH 0/3] powerpc/mm: Conditionally call H_BLOCK_REMOVE
To:     "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        mpe@ellerman.id.au, benh@kernel.crashing.org, paulus@samba.org,
        npiggin@gmail.com
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        linux-kernel@vger.kernel.org
References: <20190830120712.22971-1-ldufour@linux.ibm.com>
 <1c499131-36f2-9d89-ed4c-5cb59a08398d@linux.ibm.com>
From:   Laurent Dufour <ldufour@linux.ibm.com>
Date:   Fri, 13 Sep 2019 13:09:42 +0200
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Le 12/09/2019 à 15:44, Aneesh Kumar K.V a écrit :
> On 8/30/19 5:37 PM, Laurent Dufour wrote:
>> Since the commit ba2dd8a26baa ("powerpc/pseries/mm: call H_BLOCK_REMOVE"),
>> the call to H_BLOCK_REMOVE is always done if the feature is exhibited.
>>
>> On some system, the hypervisor may not support all the combination of
>> segment base page size and page size. When this happens the hcall is
>> returning H_PARAM, which is triggering a BUG_ON check leading to a panic.
>>
>> The PAPR document is specifying a TLB Block Invalidate Characteristics item
>> detailing which couple base page size, page size the hypervisor is
>> supporting through H_BLOCK_REMOVE. Furthermore, the characteristics are
>> also providing the size of the block the hcall could process.
>>
>> Supporting various block size seems not needed as all systems I was able to
>> play with was support an 8 addresses block size, which is the maximum
>> through the hcall. Supporting various size may complexify the algorithm in
>> call_block_remove() so unless this is required, this is not done.
>>
>> In the case of block size different from 8, a warning message is displayed
>> at boot time and that block size will be ignored checking for the
>> H_BLOCK_REMOVE support.
>>
>> Due to the minimal amount of hardware showing a limited set of
>> H_BLOCK_REMOVE supported page size, I don't think there is a need to push
>> this series to the stable mailing list.
>>
>> The first patch is initializing the penc values for each page size to an
>> invalid value to be able to detect those which have been initialized as 0
>> is a valid value.
>>
>> The second patch is reading the characteristic through the hcall
>> ibm,get-system-parameter and record the supported block size for each page
>> size.
>>
>> The third patch is changing the check used to detect the H_BLOCK_REMOVE
>> availability to take care of the base page size and page size couple.
>>
> 
> So ibm,segment-page-sizes indicates wether we support a combination of base 
> page size and actual page size. You are suggesting that the value reported 
> by that is not correct? Can you also share the early part of dmesg as below.

I'm not saying that the value reported by ibm,segment-page-sizes are 
incorrect, I'm saying that some couple are not supported by the hcall 
H_BLOCK_REMOVE.

May be should I change the second sentence by

On some system, the hypervisor may not support all the combination of 
segment base page size and page size for the hcall H_BLOCK_REMOVE. When 
this happens the hcall is returning H_PARAM, which is triggering a BUG_ON 
check leading to a panic.

Is that clear enough now ?

> 
> [    0.000000] hash-mmu: Page sizes from device-tree:
> [    0.000000] hash-mmu: base_shift=12: shift=12, sllp=0x0000, 
> avpnm=0x00000000, tlbiel=1, penc=0
> [    0.000000] hash-mmu: base_shift=12: shift=16, sllp=0x0000, 
> avpnm=0x00000000, tlbiel=1, penc=7
> [    0.000000] hash-mmu: base_shift=12: shift=24, sllp=0x0000, 
> avpnm=0x00000000, tlbiel=1, penc=56
> [    0.000000] hash-mmu: base_shift=16: shift=16, sllp=0x0110, 
> avpnm=0x00000000, tlbiel=1, penc=1
> [    0.000000] hash-mmu: base_shift=16: shift=24, sllp=0x0110, 
> avpnm=0x00000000, tlbiel=1, penc=8
> [    0.000000] hash-mmu: base_shift=24: shift=24, sllp=0x0100, 
> avpnm=0x00000001, tlbiel=0, penc=0
> [    0.000000] hash-mmu: base_shift=34: shift=34, sllp=0x0120, 
> avpnm=0x000007ff, tlbiel=0, penc=3
> 
> That shows different base page size and actual page size combination.
> 
> 
>> Laurent Dufour (3):
>>    powerpc/mm: Initialize the HPTE encoding values
>>    powperc/mm: read TLB Block Invalidate Characteristics
>>    powerpc/mm: call H_BLOCK_REMOVE when supported
>>
>>   arch/powerpc/include/asm/book3s/64/mmu.h |   3 +
>>   arch/powerpc/mm/book3s64/hash_utils.c    |   8 +-
>>   arch/powerpc/platforms/pseries/lpar.c    | 118 ++++++++++++++++++++++-
>>   3 files changed, 125 insertions(+), 4 deletions(-)
>>
> 
> 
> -aneesh