Re: [Xen-devel] [PATCH v4 2/9] xen/arm: Add more registers for saving and restoring vcpu registers

[Jaeyong Yoo <jaeyong.yoo@xxxxxxxxxxx>]

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxx [mailto:xen-devel-
> bounces@xxxxxxxxxxxxx] On Behalf Of Ian Campbell
> Sent: Friday, October 11, 2013 7:26 PM
> To: Tim Deegan
> Cc: Keir Fraser; Stefano Stabellini; Jan Beulich; Jaeyong Yoo; xen-
> devel@xxxxxxxxxxxxx
> Subject: Re: [Xen-devel] [PATCH v4 2/9] xen/arm: Add more registers for
> saving and restoring vcpu registers
> 
> On Fri, 2013-10-11 at 11:22 +0100, Tim Deegan wrote:
> > At 09:43 +0100 on 11 Oct (1381484614), Ian Campbell wrote:
> > > On Fri, 2013-10-11 at 17:30 +0900, Jaeyong Yoo wrote:
> > > > > -----Original Message-----
> > > > > From: xen-devel-bounces@xxxxxxxxxxxxx [mailto:xen-devel-
> > > > > bounces@xxxxxxxxxxxxx] On Behalf Of Ian Campbell
> > > > > Sent: Thursday, October 10, 2013 7:41 PM
> > > > > To: Jaeyong Yoo
> > > > > Cc: Tim Deegan; xen-devel@xxxxxxxxxxxxx
> > > > > Subject: Re: [Xen-devel] [PATCH v4 2/9] xen/arm: Add more
> > > > > registers for saving and restoring vcpu registers
> > > > >
> > > > > On Fri, 2013-10-04 at 13:43 +0900, Jaeyong Yoo wrote:
> > > > > > diff --git a/xen/include/public/arch-arm.h
> > > > > > b/xen/include/public/arch-arm.h index 5d359af..bf6dc9a 100644
> > > > > > --- a/xen/include/public/arch-arm.h
> > > > > > +++ b/xen/include/public/arch-arm.h
> > > > > > @@ -253,6 +253,41 @@ struct vcpu_guest_context {
> > > > > >
> > > > > >      uint32_t sctlr, ttbcr;
> > > > > >      uint64_t ttbr0, ttbr1;
> > > > > > +    uint32_t ifar, dfar;
> > > > > > +    uint32_t ifsr, dfsr;
> > > > > > +    uint32_t dacr;
> > > > > > +    uint64_t par;
> > > > > > +
> > > > > > +#ifdef CONFIG_ARM_32
> > > > >
> > > > > I'm afraid a per arch ifdef isn't allowed in the include/public
> tree.
> > > > > The interface should be identical for both 32 and 64 bit
> > > > > callers. Also think of 32-on-64 guests etc.
> > > > >
> > > > > Also, this struct is guest facing (via VCPUOP_initialise) but
> > > > > many/all of these new registers are not things which a guest
> > > > > needs to specify via a hypercall. IOW I think many of them
> > > > > should be part of some toolstack private save/restore interface.
> > > >
> > > > I see, the guest can specify something like sctlr, and ttbr/ttbcr,
> > > > and the others should be hidden inside hvm save/restore.
> > >
> > > Right, the important thing is that all that additional state is only
> > > visible to the toolstack and the hypervisor, not to guests.
> > >
> > > Actually the guest shouldn't really see this interface anyway,
> > > that's really a hold over from x86. On ARM only the toolstack really
> > > needs to use this struct.
> > >
> > > I wonder if we can drop struct vcpu_guest_context from the guest
> > > facing ABI on ARM. I see that we already don't expose
> > > VCPUOP_initialise and the only other user is
> XEN_DOMCTL_(sg)etvcpucontext.
> >
> > Does the guest need to have those on ARM? How are you arranging SMP
> > guest AP bringup?  If it's using SCI then maybe that hypercall
> > interface can be dropped.
> 
> SMP bring up is done via PSCI, which is the firmware "hypercall"
> interface, defined by ARM for bringing up physucal CPUs which we implement
> within Xen for the guests' benefit.

So, could I remove XEN_DOMCTL_(sg)etvcpucontext and use HVM save/restore for
migrating vcpu registers? 

And, if I could, arch_get_info_guest becomes dangling function and would it
be better to remove this function too? or better to keep it for symmetry?

Jaeyong

> 
> Ian
> 
> 
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