Re: [Xen-devel] [PATCH] KVM: arm64: add workaround for Cortex-A57 erratum #852523

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Mon, 2015-09-14 at 16:06 +0100, Will Deacon wrote:
> When restoring the system register state for an AArch32 guest at EL2,
> writes to DACR32_EL2 may not be correctly synchronised by Cortex-A57,
> which can lead to the guest effectively running with junk in the DACR
> and running into unexpected domain faults.

Thanks for the CC, dropping down to just the Xen folks/list and you guys.

The errata doc I've got doesn't yet cover this, so I've a few questions.

> This patch works around the issue by re-ordering our restoration of the
> AArch32 register aliases so that they happen before the AArch64 system
> registers. Ensuring that the registers are restored in this order
> guarantees that they will be correctly synchronised by the core.

Is it required that the AArch32 aliases are all restored strictly before
the AArch64 sysregs, or just that at least one sysreg is restored after
DACR32_EL2 (or a specific one?)?

The Xen ctxt switch code[0] has DACR_EL2 in the midst of it all, and
certainly followed by some sysregs, which I've got my fingers crossed
happens to be sufficient (other than maybe adding a comment).

Cheers,
Ian.

[0] 
http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/arch/arm/domain.c;h=b2bfc7d293ada3cd1695873c014e71d809c8e69d;hb=HEAD#l104
 
> 
> Cc: <stable@xxxxxxxxxxxxxxx>
> Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
> Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
> ---
>  arch/arm64/kvm/hyp.S | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/hyp.S b/arch/arm64/kvm/hyp.S
> index 3c4f641451bb..c4016d411f4a 100644
> --- a/arch/arm64/kvm/hyp.S
> +++ b/arch/arm64/kvm/hyp.S
> @@ -739,6 +739,9 @@ ENTRY(__kvm_vcpu_run)
>       // Guest context
>       add     x2, x0, #VCPU_CONTEXT
>  
> +     // We must restore the 32-bit state before the sysregs, thanks
> +     // to Cortex-A57 erratum #852523.
> +     restore_guest_32bit_state
>       bl __restore_sysregs
>  
>       skip_debug_state x3, 1f
> @@ -746,7 +749,6 @@ ENTRY(__kvm_vcpu_run)
>       kern_hyp_va x3
>       bl      __restore_debug
>  1:
> -     restore_guest_32bit_state
>       restore_guest_regs
>  
>       // That's it, no more messing around.

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