Re: [PATCH v4 17/40] KVM: arm64: Move userspace system registers into separate function

[Christoffer Dall]

On Mon, Feb 19, 2018 at 05:21:17PM +, Julien Grall wrote:
> Hi Christoffer,
> 
> On 15/02/18 21:03, Christoffer Dall wrote:
> >There's a semantic difference between the EL1 registers that control
> >operation of a kernel running in EL1 and EL1 registers that only control
> >userspace execution in EL0.  Since we can defer saving/restoring the
> >latter, move them into their own function.
> >
> >ACTLR_EL1 is not used by a VHE host, so we can move this register into
> >the EL1 state which is not saved/restored for a VHE host.
> 
> Looking at D10.2.1 (ARM DDI 0487C.a), the statement regarding the use of
> ACTLR_EL1 seems to be less strong than what you state here. It looks like it
> would be possible to have hardware where ACTLR_EL1 would still have an
> effect on host EL0. I also read the comments on the version 2 of this patch
> but I wasn't able to find what I missing.
> 
As Marc points out, I'll add a reference to the spec that says that Arm
recommends ACTLR_EL1 not having an effect on VHE host EL0.  While
potentially possible, it's unlikely, and we can fix that later if anyone
builds a platform with VHE that we care about that uses ACTLR_EL1 for
VHE host EL0.

Thanks,
-Christoffer
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Re: [PATCH v4 17/40] KVM: arm64: Move userspace system registers into separate function

[Marc Zyngier]

On Thu, 15 Feb 2018 21:03:09 +,
Christoffer Dall wrote:
> 
> There's a semantic difference between the EL1 registers that control
> operation of a kernel running in EL1 and EL1 registers that only control
> userspace execution in EL0.  Since we can defer saving/restoring the
> latter, move them into their own function.
> 
> ACTLR_EL1 is not used by a VHE host, so we can move this register into
> the EL1 state which is not saved/restored for a VHE host.

Nit: maybe worth adding a reference to the D10.2.1 comment in the
ARMv8 ARM that indicates the "recommended" behaviour of this register?

> 
> We also take this chance to rename the function saving/restoring the
> remaining system register to make it clear this function deals with
> the EL1 system registers.
> 
> Reviewed-by: Andrew Jones 
> Signed-off-by: Christoffer Dall 

Reviewed-by: Marc Zyngier 

M.

-- 
Jazz is not dead, it just smell funny.
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Re: [PATCH v4 17/40] KVM: arm64: Move userspace system registers into separate function

[Julien Grall]

Hi Christoffer,

On 15/02/18 21:03, Christoffer Dall wrote:

There's a semantic difference between the EL1 registers that control
operation of a kernel running in EL1 and EL1 registers that only control
userspace execution in EL0.  Since we can defer saving/restoring the
latter, move them into their own function.

ACTLR_EL1 is not used by a VHE host, so we can move this register into
the EL1 state which is not saved/restored for a VHE host.


Looking at D10.2.1 (ARM DDI 0487C.a), the statement regarding the use of 
ACTLR_EL1 seems to be less strong than what you state here. It looks 
like it would be possible to have hardware where ACTLR_EL1 would still 
have an effect on host EL0. I also read the comments on the version 2 of 
this patch but I wasn't able to find what I missing.


Cheers,

--
Julien Grall
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