linux-kernel - Re: [PATCH 04/12] swiotlb-xen: ensure to issue well-formed XENMEM

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Date:   Wed, 8 Sep 2021 11:16:47 +0200
From:   Jan Beulich <jbeulich@...e.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>
Subject: Re: [PATCH 04/12] swiotlb-xen: ensure to issue well-formed
 XENMEM_exchange requests

On 08.09.2021 08:57, Christoph Hellwig wrote:
> On Tue, Sep 07, 2021 at 02:05:32PM +0200, Jan Beulich wrote:
>> While the hypervisor hasn't been enforcing this, we would still better
>> avoid issuing requests with GFNs not aligned to the requested order.
>>
>> Signed-off-by: Jan Beulich <jbeulich@...e.com>
>> ---
>> I wonder how useful it is to include the alignment in the panic()
>> message. I further wonder how useful it is to wrap "bytes" in
>> PAGE_ALIGN(), when it is a multiple of a segment's size anyway (or at
>> least was supposed to be, prior to "swiotlb-xen: maintain slab count
>> properly").
>>
>> --- a/drivers/xen/swiotlb-xen.c
>> +++ b/drivers/xen/swiotlb-xen.c
>> @@ -231,10 +231,10 @@ retry:
>>  	/*
>>  	 * Get IO TLB memory from any location.
>>  	 */
>> -	start = memblock_alloc(PAGE_ALIGN(bytes), PAGE_SIZE);
>> +	start = memblock_alloc(PAGE_ALIGN(bytes), IO_TLB_SEGSIZE << IO_TLB_SHIFT);
>>  	if (!start)
>> -		panic("%s: Failed to allocate %lu bytes align=0x%lx\n",
>> -		      __func__, PAGE_ALIGN(bytes), PAGE_SIZE);
>> +		panic("%s: Failed to allocate %lu bytes align=%#x\n",
>> +		      __func__, PAGE_ALIGN(bytes), IO_TLB_SEGSIZE << IO_TLB_SHIFT);
> 
> CAn you avoid the overly long lines here?  A good way to make it more
> readable would be a variable to hold the byte count.

There already is a variable for the byte count - bytes. Did you read
the post-commit-message remark? _That's_ what I'd prefer to shorten
things. Meanwhile I can of course wrap lines; I will admit that I
failed to pay attention to line length here.

Jan