linux-kernel - Re: [Xen-devel] [PATCH 3/3] xen/swiotlb: remember having called xen_create_contiguous

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Date:   Thu, 25 Apr 2019 11:07:11 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Jan Beulich <JBeulich@...e.com>
Cc:     Stefano Stabellini <sstabellini@...nel.org>,
        iommu@...ts.linux-foundation.org,
        xen-devel <xen-devel@...ts.xenproject.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 3/3] xen/swiotlb: remember having called
 xen_create_contiguous_region()

On 25/04/2019 11:01, Jan Beulich wrote:
>>>> On 23.04.19 at 20:36, <jgross@...e.com> wrote:
>> On 23/04/2019 19:05, Stefano Stabellini wrote:
>>> On Tue, 23 Apr 2019, Juergen Gross wrote:
>>>> Instead of always calling xen_destroy_contiguous_region() in case the
>>>> memory is DMA-able for the used device, do so only in case it has been
>>>> made DMA-able via xen_create_contiguous_region() before.
>>>>
>>>> This will avoid a lot of xen_destroy_contiguous_region() calls for
>>>> 64-bit capable devices.
>>>>
>>>> As the memory in question is owned by swiotlb-xen the PG_owner_priv_1
>>>> flag of the first allocated page can be used for remembering.
>>>
>>> Although the patch looks OK, this sentence puzzles me. Why do you say
>>> that the memory in question is owned by swiotlb-xen? Because it was
>>> returned by xen_alloc_coherent_pages? Both the x86 and the Arm
>>> implementation return fresh new memory, hence, it should be safe to set
>>> the PageOwnerPriv1 flag?
>>>
>>> My concern with this approach is with the semantics of PG_owner_priv_1.
>>> Is a page marked with PG_owner_priv_1 only supposed to be used by the
>>> owner?
>>
>> The owner of the page is free to use the flag.
>>
>> Like Grant pages are marked by the grant driver using this flag. And
>> Xen page tables are using it in PV-guests for indicating a "Pinned"
>> page table.
> 
> Considering the background of the series, isn't such multi-purpose use
> of the flag a possible problem? You're already suspecting a wrong call
> into here. The function finding the flag set (but for another reason)
> might add to the confusion. But I realize there are only so many page
> flags available.

Right.

> Perhaps the freeing function should, first thing, check the handed
> space actually matches the criteria (within dma_mask and contiguous)?

Yes, I think this is a good idea.


Juergen