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 16:19:44 +0000
From:	Marc Zyngier <marc.zyngier@....com>
To:	Christoffer Dall <christoffer.dall@...aro.org>
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 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).

>>
>>>
>>> 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?

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...