Re: [Xen-devel] [PATCH v2] Livepatch for ARM 64 and 32.

[Julien Grall <julien.grall@xxxxxxx>]

On 31/08/16 16:24, Andrew Cooper wrote:
> On 31/08/16 16:09, Julien Grall wrote:
> > On 31/08/16 16:06, Konrad Rzeszutek Wilk wrote:
> > > On Wed, Aug 31, 2016 at 03:49:55PM +0100, Julien Grall wrote:
> > > > On 25/08/16 14:37, Konrad Rzeszutek Wilk wrote:
> > > > > Hey!
> > > > Hi Konrad,
> > > >
> > > > > Since v1 (and RFC):
> > > > > [https://lists.xen.org/archives/html/xen-devel/2016-08/msg01835.html]
> > > > >  - Acted on most all comments.
> > > > >  - Added ARM32 support.
> > > > >
> > > > > The patches are based on: [PATCH v4] Livepatch fixes and features
> > > > > for v4.8.
> > > > > (https://lists.xen.org/archives/html/xen-devel/2016-08/msg02705.html)
> > > > >
> > > > > And the git tree is:
> > > > >  git://xenbits.xen.org/people/konradwilk/xen.git livepatch.v4.8.v4
> > > > >
> > > > > There are two outstanding questions that should be addressed at
> > > > > some point:
> > > > >  -  #16 "livepatch: tests: Make them compile under ARM64"
> > > > >     We chatted about adding a specific CPU bit (LIVEPATCH) so that
> > > > > alternative
> > > > >     code always gets exercised. This being for both x86 and ARM.
> > > > > But this has
> > > > >     a side effect that it gets exposed to the toolstack and can
> > > > > also be
> > > > >     exposed to the guests. I think it is better if we do not do that
> > > > >     so left it to be enabled based on the most common errata (or
> > > > > feature
> > > > >     on x86).
> > > > There is no common errata on ARM. I spoke with Andrew which
> > > > mentioned that
> > > > on x86 they have architectural xen feature bits which are not
> > > > exposed to the
> > > > toolstack/guest.
> > > >
> > > > Furthermore, I think it would be unwise to let the toolstack aware
> > > > of some
> > > > errata and possibly setting/clearing them. So adding a specific
> > > > feature bit
> > > > is fine by me.
> > > So you are thinking of exposing this 'xen feature bits' that Andrew
> > > mention
> > > to be on ARM as well.
> > I only chat with Andrew about it and I have not looked at the code, so
> > I can't tell how it is on x86. I was just thinking to add LIVEPATCH
> > bit in cpufeature.h for now and be worry later when cpufeature will be
> > exposed to the toolstack for ARM.
> Sorry - I have forgotten what exactly the purpose of this bit needs to
> be, and therefore which bit of x86 I point you at.
The purpose of this bit is to be able to test the alternative with 
livepatching on any platform.
All the current feature bits are for errata and none of them are common 
enough to ensure a proper testing.
So I suggested to Konrad to introduce a feature bit LIVEPATCH that would 
be enabled unconditionally or when debug is enabled. So if you load the 
hello world payload, the alternative code will effectively be exercised.
Cheers,

--
Julien Grall

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