From: Juergen Gross <jgross@suse.com>
To: Jan Beulich <JBeulich@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
iommu@lists.linux-foundation.org,
xen-devel <xen-devel@lists.xenproject.org>,
Boris Ostrovsky <boris.ostrovsky@oracle.com>,
Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
lkml <linux-kernel@vger.kernel.org>
Subject: Re: [Xen-devel] [PATCH 3/3] xen/swiotlb: remember having called xen_create_contiguous_region()
Date: Thu, 25 Apr 2019 11:07:11 +0200 [thread overview]
Message-ID: <a1203397-9704-192a-c157-442dbeee62c5@suse.com> (raw)
On 25/04/2019 11:01, Jan Beulich wrote:
>>>> On 23.04.19 at 20:36, <jgross@suse.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
next reply other threads:[~2019-04-25 9:07 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-25 9:07 Juergen Gross [this message]
-- strict thread matches above, loose matches on Subject: below --
2019-04-23 10:54 [PATCH 0/3] xen/swiotlb: fix an issue and improve swiotlb-xen Juergen Gross
2019-04-23 10:54 ` [PATCH 3/3] xen/swiotlb: remember having called xen_create_contiguous_region() Juergen Gross
2019-04-23 17:05 ` Stefano Stabellini
2019-04-23 18:36 ` Juergen Gross
2019-04-25 9:01 ` [Xen-devel] " Jan Beulich
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=a1203397-9704-192a-c157-442dbeee62c5@suse.com \
--to=jgross@suse.com \
--cc=JBeulich@suse.com \
--cc=boris.ostrovsky@oracle.com \
--cc=iommu@lists.linux-foundation.org \
--cc=konrad.wilk@oracle.com \
--cc=linux-kernel@vger.kernel.org \
--cc=sstabellini@kernel.org \
--cc=xen-devel@lists.xenproject.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).