linux-kernel - Re: [PATCH v2 10/21] arm64: KVM: VHE: Split save/restore of sysregs shared between EL1 and EL2

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Date:	Tue, 2 Feb 2016 21:07:20 +0100
From:	Christoffer Dall <christoffer.dall@...aro.org>
To:	Marc Zyngier <marc.zyngier@....com>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, kvmarm@...ts.cs.columbia.edu
Subject: Re: [PATCH v2 10/21] arm64: KVM: VHE: Split save/restore of sysregs
 shared between EL1 and EL2

On Tue, Feb 02, 2016 at 04:19:44PM +0000, Marc Zyngier wrote:
> On 02/02/16 15:46, Christoffer Dall wrote:
> > On Tue, Feb 02, 2016 at 09:46:05AM +0000, Marc Zyngier wrote:
> >> On 01/02/16 13:54, Christoffer Dall wrote:
> >>> On Mon, Jan 25, 2016 at 03:53:44PM +0000, Marc Zyngier wrote:
> >>>> A handful of system registers are still shared between EL1 and EL2,
> >>>> even while using VHE. These are tpidr*_el[01], actlr_el1, sp0, elr,
> >>>> and spsr.
> >>>
> >>> So by shared registers you mean registers that do both have an EL0/1
> >>> version as well as an EL2 version, but where accesses aren't rewritten
> >>> transparently?
> >>
> >> No, I mean that these registers do *not* have a separate banked version.
> >> There is only a single set of registers, which have to be save/restored
> >> the old way.
> > 
> > huh, ARMv8 clearly specifies the existence of TPIDR_EL0, TPIDR_EL1, and
> > TPIDR_EL2, for example.
> > 
> > I cannot seem to find anywhere in the VHE spec that says that the
> > TPIDR_EL2 goes away.  I'm confused now.
> 
> Nothing goes away, but these registers do not get renamed either. For
> example, TPIDR_EL1 doesn't magically access TPIDR_EL2 when running at
> EL2+VHE, and there is no TPIDR_EL12 accessor either.
> 
> So TPIDR_EL1 is effectively "shared" between host and guest, and must be
> save/restored (note that the host kernel still uses TIPDR_EL1 even when
> running with VHE, and that KVM still uses TPIDR_EL2 to cache the current
> vcpu).
> 

ok, I can understand as long as we're saying a register is shared
between the host and the guest, but it was the "registers are shared
between EL1 and EL2" that threw me off.

> >>
> >>>
> >>> also, by sp0 do you mean sp_el0, and by elr you mean elr_el1, and by
> >>> spsr you mean spsr_el1 ?
> >>
> >> sp0 -> sp_el0 indeed. elr and spsr really are the guest PC and PSTATE,
> >> so I should really reword this commit message, it is utterly confusing.
> >>
> > I guess I don't understand the definition of a 'shared' register given
> > your comments here...
> 
> Does this make it clearer?
> 
yes.  You could change the host to path it when using VHE to use
TPIDR_EL2 if you wanted and store the vcpu pointer on the stack while
running the guest, but there's probably no real benefit of doing so.

I'll be shutting up now...

Thanks,
-Christoffer