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Subject: Re: [PATCH 3/3] powerpc/pseries/mm: call H_BLOCK_REMOVE
To:     Michael Ellerman <mpe@ellerman.id.au>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Cc:     aneesh.kumar@linux.ibm.com, benh@kernel.crashing.org,
        paulus@samba.org, npiggin@gmail.com
References: <1532699493-10883-1-git-send-email-ldufour@linux.vnet.ibm.com>
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 <877elcj0oa.fsf@concordia.ellerman.id.au>
From:   Laurent Dufour <ldufour@linux.vnet.ibm.com>
Date:   Thu, 16 Aug 2018 11:41:07 +0200
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On 30/07/2018 15:47, Michael Ellerman wrote:
> Hi Laurent,
> 
> Just one comment below.
> 
> Laurent Dufour <ldufour@linux.vnet.ibm.com> writes:
>> diff --git a/arch/powerpc/platforms/pseries/lpar.c b/arch/powerpc/platforms/pseries/lpar.c
>> index 96b8cd8a802d..41ed03245eb4 100644
>> --- a/arch/powerpc/platforms/pseries/lpar.c
>> +++ b/arch/powerpc/platforms/pseries/lpar.c
>> @@ -418,6 +418,73 @@ static void pSeries_lpar_hpte_invalidate(unsigned long slot, unsigned long vpn,
>>  	BUG_ON(lpar_rc != H_SUCCESS);
>>  }
>>  
>> +
>> +/*
>> + * As defined in the PAPR's section 14.5.4.1.8
>> + * The control mask doesn't include the returned reference and change bit from
>> + * the processed PTE.
>> + */
>> +#define HBLKR_AVPN		0x0100000000000000UL
>> +#define HBLKR_CTRL_MASK		0xf800000000000000UL
>> +#define HBLKR_CTRL_SUCCESS	0x8000000000000000UL
>> +#define HBLKR_CTRL_ERRNOTFOUND	0x8800000000000000UL
>> +#define HBLKR_CTRL_ERRBUSY	0xa000000000000000UL
>> +
>> +/**
>> + * H_BLOCK_REMOVE caller.
>> + * @idx should point to the latest @param entry set with a PTEX.
>> + * If PTE cannot be processed because another CPUs has already locked that
>> + * group, those entries are put back in @param starting at index 1.
>> + * If entries has to be retried and @retry_busy is set to true, these entries
>> + * are retried until success. If @retry_busy is set to false, the returned
>> + * is the number of entries yet to process.
>> + */
>> +static unsigned long call_block_remove(unsigned long idx, unsigned long *param,
>> +				       bool retry_busy)
>> +{
>> +	unsigned long i, rc, new_idx;
>> +	unsigned long retbuf[PLPAR_HCALL9_BUFSIZE];
>> +
>> +again:
>> +	new_idx = 0;
>> +	BUG_ON((idx < 2) || (idx > PLPAR_HCALL9_BUFSIZE));
> 
> I count 1 ..
> 
>> +	if (idx < PLPAR_HCALL9_BUFSIZE)
>> +		param[idx] = HBR_END;
>> +
>> +	rc = plpar_hcall9(H_BLOCK_REMOVE, retbuf,
>> +			  param[0], /* AVA */
>> +			  param[1],  param[2],  param[3],  param[4], /* TS0-7 */
>> +			  param[5],  param[6],  param[7],  param[8]);
>> +	if (rc == H_SUCCESS)
>> +		return 0;
>> +
>> +	BUG_ON(rc != H_PARTIAL);
> 
> 2 ...
> 
>> +	/* Check that the unprocessed entries were 'not found' or 'busy' */
>> +	for (i = 0; i < idx-1; i++) {
>> +		unsigned long ctrl = retbuf[i] & HBLKR_CTRL_MASK;
>> +
>> +		if (ctrl == HBLKR_CTRL_ERRBUSY) {
>> +			param[++new_idx] = param[i+1];
>> +			continue;
>> +		}
>> +
>> +		BUG_ON(ctrl != HBLKR_CTRL_SUCCESS
>> +		       && ctrl != HBLKR_CTRL_ERRNOTFOUND);
> 
> 3 ...
> 
> BUG_ON()s.
> 
> I know the code in this file is already pretty liberal with the use of
> BUG_ON() but I'd prefer if we don't make it any worse.

The first one is clearly not required. But I would keep the following twos
because this call is not expected to fail except if there is a discrepancy
between the linux kernel HASH views and the hypervisor's one, which could be
dramatic in the consequences.

> 
> Given this is an optimisation it seems like we should be able to fall
> back to the existing implementation in the case of error (which will
> probably then BUG_ON() ????)

I don't think falling back to the H_BULK call will be helpfull since it is
doing the same so the same errors are expected. Furthermore, this hcall can do
a partial work which means complex code to fallback on H_BULK as we should
identify to already processed entries.

> If there's some reason we can't then I guess I can live with it.

I'm proposing to send a new series with _only_ 2 calls to BUG_ON().

Furthermore this patch is not correct on the way the huge pages are managed. I
was too hurry to push it last time.

Cheers,
Laurent.