From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
To: "Roger Pau Monné" <roger.pau@citrix.com>
Cc: xen-devel@lists.xenproject.org, iwj@xenproject.org, wl@xen.org,
anthony.perard@citrix.com, jbeulich@suse.com,
andrew.cooper3@citrix.com, jun.nakajima@intel.com,
kevin.tian@intel.com
Subject: Re: [PATCH v2 2/4] x86: Introduce MSR_UNHANDLED
Date: Mon, 22 Feb 2021 16:19:04 -0500 [thread overview]
Message-ID: <ce2ef7a3-0583-ffff-182a-0ab078f45b82@oracle.com> (raw)
In-Reply-To: <YDOQvU1h8zpOv5PH@Air-de-Roger>
On 2/22/21 6:08 AM, Roger Pau Monné wrote:
> On Fri, Feb 19, 2021 at 09:56:32AM -0500, Boris Ostrovsky wrote:
>> On 2/18/21 5:51 AM, Roger Pau Monné wrote:
>>> On Wed, Jan 20, 2021 at 05:49:10PM -0500, Boris Ostrovsky wrote:
>>>> When toolstack updates MSR policy, this MSR offset (which is the last
>>>> index in the hypervisor MSR range) is used to indicate hypervisor
>>>> behavior when guest accesses an MSR which is not explicitly emulated.
>>> It's kind of weird to use an MSR to store this. I assume this is done
>>> for migration reasons?
>>
>> Not really. It just seemed to me that MSR policy is the logical place to do that. Because it *is* a policy of how to deal with such accesses, isn't it?
> I agree that using the msr_policy seems like the most suitable place
> to convey this information between the toolstack and Xen. I wonder if
> it would be fine to have fields in msr_policy that don't directly
> translate into an MSR value?
We have xen_msr_entry_t.flags that we can use when passing policy array back and forth. Then we can ignore xen_msr_entry_t.idx for that entry (although in earlier version of this series Jan preferred to use idx and leave flags alone).
>
> But having such a list of ignored MSRs in msr_policy makes the whole
> get/set logic a bit weird, as the user would have to provide a buffer
> in order to get the list of ignored MSRs.
If we go with ranges/lists of ignored MSRs then we will need to have ignore_msrs as a rangeset in msr_policy, not as (current) uint8. And xen_msr_entry_t will need to have a range as opposed to single index. Or maybe I don't understand what you are referring to as get/set logic.
But I would like to make sure we really want to support such ranges, I am a little concerned about over-engineering this (especially wrt migration, I think it will need marshalling/unmarshalling)
>>> Isn't is possible to convey this data in the xl migration stream
>>> instead of having to pack it with MSRs?
>>
>> I haven't looked at this but again --- the feature itself has nothing to do with migration. The fact that folding it into policy makes migration of this information "just work" is just a nice side benefit of this implementation.
> IMO it feels slightly weird that we have to use a MSR (MSR_UNHANDLED)
> to store this option, seems like wasting an MSR index when there's
> really no need for it to be stored in an MSR, as we don't expose it to
> guests.
>
> It would seem more natural for such option to live in arch_domain as a
> rangeset for example.
>
> Maybe introduce a new DOMCTL to set it?
>
> #define XEN_DOMCTL_msr_set_ignore ...
> struct xen_domctl_msr_set_ignore {
> uint32_t index;
> uint32_t size;
> };
That will work too but this is adding 2 new domctls (I think we will need a "get" too) whereas with policy we use existing interface.
-boris
next prev parent reply other threads:[~2021-02-22 21:19 UTC|newest]
Thread overview: 53+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-20 22:49 [PATCH v2 0/4] Permit fault-less access to non-emulated MSRs Boris Ostrovsky
2021-01-20 22:49 ` [PATCH v2 1/4] xl: Add support for ignore_msrs option Boris Ostrovsky
2021-01-21 14:56 ` Wei Liu
2021-01-21 22:43 ` Boris Ostrovsky
2021-01-22 9:52 ` Julien Grall
2021-01-22 18:28 ` Boris Ostrovsky
2021-01-22 18:33 ` Julien Grall
2021-01-22 18:39 ` Boris Ostrovsky
2021-01-22 20:42 ` Julien Grall
2021-02-18 10:42 ` Roger Pau Monné
2021-02-18 11:54 ` Jan Beulich
2021-02-18 15:52 ` Roger Pau Monné
2021-02-18 15:57 ` Jan Beulich
2021-02-19 14:50 ` Boris Ostrovsky
2021-02-22 10:24 ` Roger Pau Monné
2021-02-22 10:33 ` Jan Beulich
2021-01-20 22:49 ` [PATCH v2 2/4] x86: Introduce MSR_UNHANDLED Boris Ostrovsky
2021-01-22 11:51 ` Jan Beulich
2021-01-22 18:56 ` Boris Ostrovsky
2021-02-02 17:01 ` Boris Ostrovsky
2021-02-18 10:51 ` Roger Pau Monné
2021-02-19 14:56 ` Boris Ostrovsky
2021-02-22 11:08 ` Roger Pau Monné
2021-02-22 21:19 ` Boris Ostrovsky [this message]
2021-02-23 7:57 ` Jan Beulich
2021-02-23 9:34 ` Roger Pau Monné
2021-02-23 10:15 ` Jan Beulich
2021-02-23 12:17 ` Roger Pau Monné
2021-02-23 13:23 ` Jan Beulich
2021-02-23 15:39 ` Boris Ostrovsky
2021-02-23 16:10 ` Jan Beulich
2021-02-23 18:00 ` Roger Pau Monné
2021-02-23 16:11 ` Roger Pau Monné
2021-02-23 16:40 ` Boris Ostrovsky
2021-02-23 18:02 ` Roger Pau Monné
2021-02-23 18:45 ` Boris Ostrovsky
2021-01-20 22:49 ` [PATCH v2 3/4] x86: Allow non-faulting accesses to non-emulated MSRs if policy permits this Boris Ostrovsky
2021-01-22 12:51 ` Jan Beulich
2021-01-22 19:52 ` Boris Ostrovsky
2021-01-25 10:22 ` Jan Beulich
2021-01-25 18:42 ` Boris Ostrovsky
2021-01-26 9:05 ` Jan Beulich
2021-01-26 16:02 ` Boris Ostrovsky
2021-01-26 16:35 ` Jan Beulich
2021-02-18 11:24 ` Roger Pau Monné
2021-02-18 11:57 ` Jan Beulich
2021-02-18 15:53 ` Roger Pau Monné
2021-01-20 22:49 ` [PATCH v2 4/4] tools/libs: Apply MSR policy to a guest Boris Ostrovsky
2021-01-21 14:58 ` Wei Liu
2021-01-22 9:56 ` Julien Grall
2021-01-22 18:35 ` Boris Ostrovsky
2021-02-18 11:48 ` Roger Pau Monné
2021-02-19 14:57 ` Boris Ostrovsky
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