From: David Vrabel <david.vrabel@citrix.com>
To: Wei Liu <wei.liu2@citrix.com>, David Vrabel <david.vrabel@citrix.com>
Cc: sstabellini@kernel.org,
Paulina Szubarczyk <paulinaszubarczyk@gmail.com>,
ian.jackson@eu.citrix.com, anthony.perard@citrix.com,
xen-devel@lists.xenproject.org, roger.pau@citrix.com
Subject: Re: [PATCH v3 1/2] Interface for grant copy operation in libs.
Date: Wed, 22 Jun 2016 13:37:50 +0100 [thread overview]
Message-ID: <576A869E.6090400@citrix.com> (raw)
In-Reply-To: <20160622112118.GB1790@citrix.com>
On 22/06/16 12:21, Wei Liu wrote:
> On Wed, Jun 22, 2016 at 10:37:24AM +0100, David Vrabel wrote:
>> On 22/06/16 09:38, Paulina Szubarczyk wrote:
>>> In a linux part an ioctl(gntdev, IOCTL_GNTDEV_GRANT_COPY, ..)
>>> system call is invoked. In mini-os the operation is yet not
>>> implemented. For other OSs there is a dummy implementation.
>> [...]
>>> --- a/tools/libs/gnttab/linux.c
>>> +++ b/tools/libs/gnttab/linux.c
>>> @@ -235,6 +235,51 @@ int osdep_gnttab_unmap(xengnttab_handle *xgt,
>>> return 0;
>>> }
>>>
>>> +int osdep_gnttab_grant_copy(xengnttab_handle *xgt,
>>> + uint32_t count,
>>> + xengnttab_grant_copy_segment_t *segs)
>>> +{
>>> + int i, rc;
>>> + int fd = xgt->fd;
>>> + struct ioctl_gntdev_grant_copy copy;
>>> +
>>> + copy.segments = calloc(count, sizeof(struct ioctl_gntdev_grant_copy_segment));
>>> + copy.count = count;
>>> + for (i = 0; i < count; i++)
>>> + {
>>> + copy.segments[i].flags = segs[i].flags;
>>> + copy.segments[i].len = segs[i].len;
>>> + if (segs[i].flags == GNTCOPY_dest_gref)
>>> + {
>>> + copy.segments[i].dest.foreign.ref = segs[i].dest.foreign.ref;
>>> + copy.segments[i].dest.foreign.domid = segs[i].dest.foreign.domid;
>>> + copy.segments[i].dest.foreign.offset = segs[i].dest.foreign.offset;
>>> + copy.segments[i].source.virt = segs[i].source.virt;
>>> + }
>>> + else
>>> + {
>>> + copy.segments[i].source.foreign.ref = segs[i].source.foreign.ref;
>>> + copy.segments[i].source.foreign.domid = segs[i].source.foreign.domid;
>>> + copy.segments[i].source.foreign.offset = segs[i].source.foreign.offset;
>>> + copy.segments[i].dest.virt = segs[i].dest.virt;
>>> + }
>>> + }
>>> +
>>> + rc = ioctl(fd, IOCTL_GNTDEV_GRANT_COPY, ©);
>>> + if (rc)
>>> + {
>>> + GTERROR(xgt->logger, "ioctl GRANT COPY failed %d ", errno);
>>> + }
>>> + else
>>> + {
>>> + for (i = 0; i < count; i++)
>>> + segs[i].status = copy.segments[i].status;
>>> + }
>>> +
>>> + free(copy.segments);
>>> + return rc;
>>> +}
>>
>> I know Wei asked for this but you've replaced what should be a single
>> pointer assignment with a memory allocation and two loops over all the
>> segments.
>>
>> This is a hot path and the two structures (the libxengnttab one and the
>> Linux kernel one) are both part of their respective ABIs and won't
>> change so Wei's concern that they might change in the future is unfounded.
>>
>
> The fundamental question is: will the ABI between the library and the
> kernel ever go mismatch?
>
> My answer is "maybe". My rationale is that everything goes across
> boundary of components need to be considered with caution. And I tend to
> assume the worst things will happen.
>
> To guarantee that they will never go mismatch is to have
>
> typedef ioctl_gntdev_grant_copy_segment xengnttab_grant_copy_segment_t;
>
> But that's not how the code is written.
>
> I would like to hear a third opinion. Is my concern unfounded? Am I too
> cautious? Is there any compelling argument that I missed?
>
> Somewhat related, can we have some numbers please? It could well be the
> cost of the two loops is much cheaper than whatever is going on inside
> the kernel / hypervisor. And it could turn out that the numbers render
> this issue moot.
I did some (very) adhoc measurements and with the worst case of single
short segments for each ioctl, the optimized version of
osdep_gnttab_grant_copy() looks to be ~5% faster.
This is enough of a difference that we should use the optimized version.
The unoptimized version also adds an additional failure path (the
calloc) which would be best avoided.
David
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next prev parent reply other threads:[~2016-06-22 12:38 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-06-22 8:38 [PATCH v3 0/2] qemu-qdisk: Implementation of grant copy operation Paulina Szubarczyk
2016-06-22 8:38 ` [PATCH v3 1/2] Interface for grant copy operation in libs Paulina Szubarczyk
2016-06-22 9:37 ` David Vrabel
2016-06-22 9:53 ` Paulina Szubarczyk
2016-06-22 11:24 ` Wei Liu
2016-06-22 14:19 ` Paulina Szubarczyk
2016-06-22 11:21 ` Wei Liu
2016-06-22 12:37 ` David Vrabel [this message]
2016-06-22 13:29 ` Wei Liu
2016-06-22 13:52 ` David Vrabel
2016-06-22 14:52 ` Wei Liu
2016-06-22 16:49 ` Wei Liu
2016-07-06 15:49 ` Roger Pau Monné
2016-07-05 16:27 ` George Dunlap
2016-07-08 13:18 ` Wei Liu
2016-07-13 9:12 ` Wei Liu
2016-06-22 8:38 ` [PATCH v3 2/2] qdisk - hw/block/xen_disk: grant copy implementation Paulina Szubarczyk
2016-07-13 12:34 ` Paulina Szubarczyk
2016-07-14 10:37 ` Wei Liu
2016-07-15 10:28 ` Paulina Szubarczyk
2016-07-15 11:15 ` Wei Liu
2016-07-15 17:11 ` Anthony PERARD
2016-07-19 10:16 ` Paulina Szubarczyk
2016-07-15 16:55 ` Anthony PERARD
2016-07-19 10:51 ` Paulina Szubarczyk
2016-07-19 9:12 ` Roger Pau Monné
2016-07-19 10:12 ` Paulina Szubarczyk
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