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From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
To: Jan Beulich <JBeulich@suse.com>
Cc: wei.liu2@citrix.com, Ian Campbell <ian.campbell@citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	ian.jackson@eu.citrix.com, mpohlack@amazon.de,
	xen-devel@lists.xenproject.org, dgdegra@tycho.nsa.gov
Subject: Re: [PATCH v1 2/4] xen-version: Add third parameter (len) to the do_version hypercall.
Date: Wed, 28 Oct 2015 13:55:40 -0400	[thread overview]
Message-ID: <20151028175540.GA11004@l.oracle.com> (raw)
In-Reply-To: <5617ED8A02000078000A9C4D@prv-mh.provo.novell.com>

On Fri, Oct 09, 2015 at 08:38:34AM -0600, Jan Beulich wrote:
> >>> On 09.10.15 at 14:46, <ian.campbell@citrix.com> wrote:
> > On Fri, 2015-10-09 at 13:29 +0100, Andrew Cooper wrote:
> >> On 09/10/15 09:25, Jan Beulich wrote:
> >> > > > > On 09.10.15 at 04:56, <konrad.wilk@oracle.com> wrote:
> >> > > All existing commands ignore the parameter so this does
> >> > > not break the ABI.
> >> > Does it not? What about the debug mode clobbering of hypercall
> >> > argument registers?
> >> 
> >> That is an implementation detail of the hypervisor.  It is irrelevant to
> >> guests whether Xen chooses to clobber the spare registers or not.
> > 
> > Or in other words the effect here is to clobber one _less_ register, and
> > the guest cannot have been relying on a register getting so clobbered (if
> > nothing else it doesn't happen in debug=n builds).
> 
> No, the one less register clobbered is in the first clobbering phase,
> where _unused_ inputs get clobbered (for hypervisor internal
> consumption). The second clobbering phase destroys all _used_
> input registers' contents (the guest visible values), and _this_ is
> what results in ABI breakage (because callers assuming the
> hypercall to take two arguments assume that the 3rd argument
> register will retain its contents.

Thanks for explaining it! I see my patch missed the change to
hypercall_table and along with compiling with debug=y it all worked
so I didn't get the 'len' parameter to be clobbered.

Ugh.

Then the right way to make this work would be to only clobber
the third argument if the XENVER_buildid command was used? And
preserve the third argument only if XENVER_buildid command was used.

And not clobber third argument in all other cases. That would
require some nasty tweaking of entry.S.

Ugh. I think going to the original idea of just having an
xen_build_id_t[1024] would be the easiest.

Or I can do a structure:

struct xen_build_id_t {
	uint32_t	len; /* IN: size of the buffer. */
	uint32_t 	_pad; /* IN: MUST be zero. */
	XEN_GUEST_HANDLE_64(char) buf; /* OUT: Buffer with build_id. */
}

Any preference? The 'xen_build_id_t[1024]' looks nicer.
> 
> Jan
> 

  parent reply	other threads:[~2015-10-28 17:55 UTC|newest]

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-10-09  2:56 [PATCH v1] Add build-id to XENVER hypercall Konrad Rzeszutek Wilk
2015-10-09  2:56 ` [PATCH v1 1/4] xsm/libxl/xen_version: Add XSM for some of the xen_version commands Konrad Rzeszutek Wilk
2015-10-09  9:31   ` Ian Campbell
2015-10-30 10:24     ` Martin Pohlack
2015-10-09 12:20   ` Andrew Cooper
2015-10-30 10:24   ` Martin Pohlack
2015-10-09  2:56 ` [PATCH v1 2/4] xen-version: Add third parameter (len) to the do_version hypercall Konrad Rzeszutek Wilk
2015-10-09  8:25   ` Jan Beulich
2015-10-09 12:29     ` Andrew Cooper
2015-10-09 12:46       ` Ian Campbell
2015-10-09 12:58         ` Andrew Cooper
2015-10-09 14:38         ` Jan Beulich
2015-10-09 14:48           ` Ian Campbell
2015-10-28 17:55           ` Konrad Rzeszutek Wilk [this message]
2015-10-28 18:34             ` Andrew Cooper
2015-10-28 18:58               ` Konrad Rzeszutek Wilk
2015-10-29  9:06               ` Jan Beulich
2015-10-09  2:56 ` [PATCH v1 3/4] XENVER_build_id: Provide ld-embedded build-ids Konrad Rzeszutek Wilk
2015-10-09  9:35   ` Ian Campbell
2015-10-09 11:40   ` Martin Pohlack
2015-10-09 12:47   ` Andrew Cooper
2015-10-09 15:18   ` Jan Beulich
2016-01-06 18:07     ` Konrad Rzeszutek Wilk
2015-10-09  2:56 ` [PATCH v1 4/4] libxl: info: Display build_id of the hypervisor Konrad Rzeszutek Wilk
2015-10-09  9:36   ` Ian Campbell
2015-10-09 12:59   ` Andrew Cooper
2015-10-09 13:06     ` Ian Campbell
2015-10-09 13:11       ` Andrew Cooper
2015-10-09 13:14       ` Ian Campbell
2015-10-09 13:16   ` Ian Campbell
2015-10-09  8:17 ` [PATCH v1] Add build-id to XENVER hypercall Jan Beulich
2015-10-09 12:15   ` Andrew Cooper
2015-10-09 13:25     ` Konrad Rzeszutek Wilk
2015-10-09 15:14       ` Jan Beulich
2015-10-28 15:42         ` Konrad Rzeszutek Wilk
2015-10-28 19:00           ` Konrad Rzeszutek Wilk
2015-10-29  8:55             ` Jan Beulich
2015-10-29 19:47               ` Konrad Rzeszutek Wilk
2015-10-30  8:11                 ` Jan Beulich
2015-10-09 14:32     ` Jan Beulich

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