Re: Runtime services support for Xen on ARM

From: Ian Campbell <ian.campbell@citrix.com>
To: Stefano Stabellini <stefano.stabellini@eu.citrix.com>,
	Shannon Zhao <zhaoshenglong@huawei.com>
Cc: xen-devel@lists.xensource.com,
	Shannon Zhao <shannon.zhao@linaro.org>,
	Christoffer Dall <christoffer.dall@linaro.org>
Subject: Re: Runtime services support for Xen on ARM
Date: Tue, 10 Nov 2015 12:36:28 +0000	[thread overview]
Message-ID: <1447158988.9407.30.camel@citrix.com> (raw)
In-Reply-To: <alpine.DEB.2.02.1511101210580.5676@kaball.uk.xensource.com>

On Tue, 2015-11-10 at 12:26 +0000, Stefano Stabellini wrote:
> CC'ing xen-devel and Jan
> 
> On Tue, 10 Nov 2015, Shannon Zhao wrote:
> > Hi Stefano,
> > 
> > I'm working on adding Runtime services support at Xen side. Most of
> > work
> > is adding the ARM part in xen/common/efi/runtime.c.
> > 
> > There is one problem which block me. That is how to implement
> > efi_rs_enter() and efi_rs_leave() for ARM, since I think current
> > implementation is x86 specific and won't work on ARM. Also the
> > rtc_lock.
> > 
> > Could you give some suggestion? Thanks!
> 
> efi_rs_enter() and efi_rs_leave() look very PV x86 specific. It is
> possible that we don't actually need to do anything at all on ARM, aside
> from refactoring the code. Jan?

I think these functions derive from the USE_SET_VIRTUAL_ADDRESS_MAP option.
If that is set we call efi_rs->SetVirtualAddressMap to tell the f/w about
our virtual address layout and can then call rs directly with no faff.

Unfortunately SetVirtualAddressMap is incompatible with kexec (I think you
can only call it once), so we have this USE_SET_VIRTUAL_ADDRESS_MAP option.

Without USE_SET_VIRTUAL_ADDRESS_MAP we need to switch to a set of page
tables which are "OK" for calling the runtime services. I'm not sure what
"OK" means here -- but I suspect it means "1:1" in some part.

So sadly I think we do _eventually_ need to support this mode (for kexec),
which would mean quite a bit of refactoring (since the relevant code
in xen/common/efi/boot.c has some x86-isms).

But right now, since we do not yet support kexec,  I think we could get the
ball rolling wrt RS support by setting USE_SET_VIRTUAL_ADDRESS_MAP on ARM
and dodging the need to make efi_rs_enter() work right now. (IOW make it
the problem of whomever wants kexec support...)

Unless there are issues other than kexec with SetVirtualAddressMap which
I'm not thinking of now?

> Similarly we don't have an rtc_lock spinlock today and it is not
> actually needed, but for the sake of keeping the common code common, I
> would just introduce rtc_lock on ARM too.

Agreed.

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