From: Wei Liu <wl@xen.org>
To: Jan Beulich <jbeulich@suse.com>
Cc: "Wei Liu" <liuwe@microsoft.com>, "Wei Liu" <wl@xen.org>,
"Paul Durrant" <paul@xen.org>,
"Andrew Cooper" <andrew.cooper3@citrix.com>,
"Michael Kelley" <mikelley@microsoft.com>,
"Xen Development List" <xen-devel@lists.xenproject.org>,
"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH v3 3/5] x86/hyperv: provide percpu hypercall input page
Date: Tue, 7 Jan 2020 17:27:20 +0000 [thread overview]
Message-ID: <20200107172720.onwjgb7x2hsrrivz@debian> (raw)
In-Reply-To: <af2dd5b5-f5a4-6fe3-5654-6d84739e6d92@suse.com>
On Tue, Jan 07, 2020 at 06:08:19PM +0100, Jan Beulich wrote:
> On 07.01.2020 17:33, Wei Liu wrote:
> > On Mon, Jan 06, 2020 at 11:27:18AM +0100, Jan Beulich wrote:
> >> On 05.01.2020 17:47, Wei Liu wrote:
> >>> Hyper-V's input / output argument must be 8 bytes aligned an not cross
> >>> page boundary. The easiest way to satisfy those requirements is to use
> >>> percpu page.
> >>
> >> I'm not sure "easiest" is really true here. Others could consider adding
> >> __aligned() attributes as easy or even easier (by being even more
> >> transparent to use sites). Could we settle on "One way ..."?
> >
> > Do you mean something like
> >
> > struct foo __aligned(8);
>
> If this is in a header and ...
>
> > hv_do_hypercall(OP, virt_to_maddr(&foo), ...);
>
> ... this in actual code, then yes.
>
> > ?
> >
> > I don't think this is transparent to user sites. Plus, foo is on stack
> > which is 1) difficult to get its maddr,
>
> It being on the stack may indeed complicate getting its machine address
> (if not now, then down the road) - valid point.
>
> > 2) may cross page boundary.
>
> The __aligned() of course needs to be large enough to avoid this
> happening.
For this alignment to be large enough, it will need to be of PAGE_SIZE,
right? Wouldn't that blow up Xen's stack easily? Given we only have two
pages for that.
In light of these restrictions, the approach I take in the original
patch should be okay.
I'm fine with changing the wording to "One way ..." -- if that's the
only objection you have after this mail.
>
> >> Also, while looking at this I notice that - despite my earlier
> >> comment when giving the respective, sort-of-conditional ack -
> >> there are (still) many apparently pointless __packed attributes
> >> in hyperv-tlfs.h. Care to comment on this?
> >
> > Again, that's a straight import from Linux. I tried not to deviate too
> > much. A commit in Linux (ec084491727b0) claims "compiler can add
> > alignment padding to structures or reorder struct members for
> > randomization and optimization".
>
> Would a compiler doing so (without explicitly being told to) even
> be in line with the C spec? I'd buy such a claim only if I see an
> example proving it.
>
> > I just checked all the packed structures. They seem to have all the
> > required manual paddings already. I can only assume they tried to erred
> > on the safe side.
>
> And you surely recall we had to remove quite a few instances of
> __packed for gcc 9 compatibility?
Fair enough. I will write a patch to drop those __packed attributes.
Wei.
>
> Jan
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next prev parent reply other threads:[~2020-01-07 17:27 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-05 16:47 [Xen-devel] [PATCH v3 0/5] More Hyper-V infrastructure Wei Liu
2020-01-05 16:47 ` [Xen-devel] [PATCH v3 1/5] x86/hyperv: setup hypercall page Wei Liu
2020-01-05 17:37 ` Andrew Cooper
2020-01-05 21:45 ` Wei Liu
2020-01-05 21:57 ` Andrew Cooper
2020-01-07 15:42 ` Wei Liu
2020-01-08 17:43 ` Wei Liu
2020-01-08 17:53 ` Andrew Cooper
2020-01-09 13:48 ` Wei Liu
2020-01-05 16:47 ` [Xen-devel] [PATCH v3 2/5] x86/hyperv: provide Hyper-V hypercall functions Wei Liu
2020-01-05 19:08 ` Andrew Cooper
2020-01-05 21:22 ` Wei Liu
2020-01-05 22:06 ` Andrew Cooper
2020-01-07 16:17 ` Wei Liu
2020-01-16 19:14 ` Andrew Cooper
2020-01-16 14:54 ` Wei Liu
2020-01-06 9:38 ` Jan Beulich
2020-01-07 16:21 ` Wei Liu
2020-01-05 16:47 ` [Xen-devel] [PATCH v3 3/5] x86/hyperv: provide percpu hypercall input page Wei Liu
2020-01-06 10:27 ` Jan Beulich
2020-01-07 16:33 ` Wei Liu
2020-01-07 16:45 ` Michael Kelley
2020-01-08 10:57 ` Jan Beulich
2020-01-07 17:08 ` Jan Beulich
2020-01-07 17:27 ` Wei Liu [this message]
2020-01-08 10:55 ` Jan Beulich
2020-01-08 15:54 ` Wei Liu
2020-01-05 16:48 ` [Xen-devel] [PATCH v3 4/5] x86/hyperv: retrieve vp_index from Hyper-V Wei Liu
2020-01-06 9:59 ` Paul Durrant
2020-01-06 10:31 ` Jan Beulich
2020-01-07 16:34 ` Wei Liu
2020-01-05 16:48 ` [Xen-devel] [PATCH v3 5/5] x86/hyperv: setup VP assist page Wei Liu
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