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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

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.


> 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.