From: Razvan Cojocaru <rcojocaru@bitdefender.com>
To: Tamas K Lengyel <tamas@tklengyel.com>,
Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Kevin Tian <kevin.tian@intel.com>,
"wei.liu2@citrix.com" <wei.liu2@citrix.com>,
Jan Beulich <jbeulich@suse.com>,
Ian Jackson <ian.jackson@eu.citrix.com>,
Xen-devel <xen-devel@lists.xen.org>,
Jun Nakajima <jun.nakajima@intel.com>, Keir Fraser <keir@xen.org>
Subject: Re: [PATCH V2] vm_event: Allow subscribing to write events for specific MSR-s
Date: Thu, 14 Apr 2016 12:37:55 +0300 [thread overview]
Message-ID: <570F64F3.7030604@bitdefender.com> (raw)
In-Reply-To: <CABfawh=XJLeysnZnq5WuF28Fn90QU5bZffo7dxLYdvrNnvtMfA@mail.gmail.com>
On 04/13/2016 06:05 PM, Tamas K Lengyel wrote:
>
>
> On Wed, Apr 13, 2016 at 9:01 AM, Andrew Cooper
> <andrew.cooper3@citrix.com <mailto:andrew.cooper3@citrix.com>> wrote:
>
> On 13/04/16 15:56, Razvan Cojocaru wrote:
> > On 04/13/2016 05:52 PM, Tamas K Lengyel wrote:
> >> >> diff --git a/xen/include/public/domctl.h
> >> b/xen/include/public/domctl.h
> >> >> index 2457698..875c09a 100644
> >> >> --- a/xen/include/public/domctl.h
> >> >> +++ b/xen/include/public/domctl.h
> >> >> @@ -1107,8 +1107,7 @@ struct xen_domctl_monitor_op {
> >> >> } mov_to_cr;
> >> >>
> >> >> struct {
> >> >> - /* Enable the capture of an extended set of
> MSRs */
> >> >> - uint8_t extended_capture;
> >> >> + uint32_t msr;
> >> >
> >> > Whoa there. Isn't it expanding the structure? Will this be
> backwards
> >> > compatible? What if somebody is using an older version of
> xen-access
> >> > against this hypervisor? Will they work?
> >> >
> >> > Perhaps this should have a new struct / sub-ops? And the old
> >> > 'mov_to_msr' will just re-use this new fangled code?
> >>
> >> In addition to Andrew's comments, I think simply changing
> >> VM_EVENT_INTERFACE_VERSION should be enough for
> xen-access-like clients
> >> to figure out the incompatibility.
> >>
> >>
> >>
> >> This is an independent system from VM_EVENT, so IMHO the two
> shouldn't
> >> be mixed. The union size right now is 24-bits so if a uint16_t is
> enough
> >> for the bitmask that should be used instead. That way we don't end up
> >> growing the struct size.
> > Right. Well, MSR-s seem to be passed around as 32-bit unsigned
> integers
> > everywhere in the Xen source code, so unless that also needs
> correcting
> > then unfortunately it'll have to grow.
>
> MSR indices are always 32bits wide, as they live specifically in %ecx
> when encoded for instructions.
>
> Only 2K MSRs are currently specified in hardware, with some extra ones
> in the hypervisor range, but this doesn't mean that list won't grow in
> the future.
>
>
> Yea, well then we need to introduce a new struct with a new subop to
> pass the bitmask. I guess its a lesson in ABI design to leave some
> wiggle room for future-proofing it (my bad). So I guess we can introduce
> XEN_DOMCTL_MONITOR_OP_ENABLE_V2 and struct xen_domctl_monitor_op_v2
> where say expand the union to uint64_t just in case?
I can do that, but it would seem that this is somewhat at odds with
Andrew Cooper's perspective - he has stated that it's within the rules
and the domctl can be changed without there being the need for
XEN_DOMCTL_MONITOR_OP_ENABLE_V2. So this should be clarified, please,
otherwise I'm incurring the risk of changing the code only to have to
revert it later.
Thanks,
Razvan
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next prev parent reply other threads:[~2016-04-14 9:37 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-13 5:11 [PATCH V2] vm_event: Allow subscribing to write events for specific MSR-s Razvan Cojocaru
2016-04-13 9:47 ` Konrad Rzeszutek Wilk
2016-04-13 10:07 ` Andrew Cooper
2016-04-13 11:57 ` Razvan Cojocaru
2016-04-13 14:52 ` Tamas K Lengyel
2016-04-13 14:56 ` Razvan Cojocaru
2016-04-13 15:01 ` Andrew Cooper
2016-04-13 15:05 ` Tamas K Lengyel
2016-04-14 9:37 ` Razvan Cojocaru [this message]
2016-04-14 15:20 ` Jan Beulich
2016-04-14 15:33 ` Tamas K Lengyel
2016-04-14 15:37 ` Razvan Cojocaru
2016-04-13 14:50 ` Tamas K Lengyel
2016-04-13 14:52 ` Razvan Cojocaru
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