Re: [PATCH] xen/arm: Add workaround for Cortex-A53 erratum #845719

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 9 Dec 2020, Michal Orzel wrote:
> On 09.12.2020 02:34, Stefano Stabellini wrote:
> > On Tue, 8 Dec 2020, Julien Grall wrote:
> >> On 08/12/2020 14:38, Bertrand Marquis wrote:
> >>> Hi Julien,
> >>>
> >>>> On 8 Dec 2020, at 09:47, Julien Grall <julien@xxxxxxx> wrote:
> >>>>
> >>>> Hi,
> >>>>
> >>>> On 08/12/2020 07:23, Michal Orzel wrote:
> >>>>> When executing in aarch32 state at EL0, a load at EL0 from a
> >>>>> virtual address that matches the bottom 32 bits of the virtual address
> >>>>> used by a recent load at (aarch64) EL1 might return incorrect data.
> >>>>> The workaround is to insert a write of the contextidr_el1 register
> >>>>> on exception return to an aarch32 guest.
> >>>>
> >>>> I am a bit confused with this comment. In the previous paragraph, you are
> >>>> suggesting that the problem is an interaction between EL1 AArch64 and EL0
> >>>> AArch32. But here you seem to imply the issue only happen when running a
> >>>> AArch32 guest.
> >>>>
> >>>> Can you clarify it?
> >>>
> >>> This can happen when switching from an aarch64 guest to an aarch32 guest 
> >>> so
> >>> not only when there is interaction.
> > 
> > Just to confirm: it cannot happen when switching from aarch64 *EL2* to
> > aarch32 EL0/1, right?  Because that happens all the time in Xen.
> > 
> > 
> No it cannot. It can only happen when switching from aarch64 EL1 to aarch32 
> EL0.

Excellent, thanks for checking


> >> Right, but the context switch will write to CONTEXTIDR_EL1. So this case
> >> should already be handled.
> >>
> >> Xen will never switch from AArch64 EL1 to AArch32 EL0 without a context 
> >> switch
> >> (the inverse can happen if we inject an exception to the guest).
> >>
> >> Reading the Cortex-A53 SDEN, it sounds like this is an OS and not 
> >> Hypervisor
> >> problem. In fact, Linux only seems to workaround it when switching in the 
> >> OS
> >> side rather than the hypervisor.
> >>
> >> Therefore, I am not sure to understand why we need to workaroud it in Xen.
> > 
> > It looks like Julien is right in regards to the "aarch64 EL1 to aarch32
> > EL0" issue.
> > 
> Yes I agree. I missed the fact that there is a write to CONTEXTIDR_EL1
> in 'ctxt_switch_to'. Let's abandon this.

Job done :-)