Re: [PATCH] arm64: KVM: Take S1 walks into account when determining S2 write faults

[Marc Zyngier <marc.zyngier@arm.com>]

On 17/10/16 11:20, Will Deacon wrote:
> On Thu, Sep 29, 2016 at 09:14:32PM +0200, Christoffer Dall wrote:
>> On Thu, Sep 29, 2016 at 12:37:01PM +0100, Will Deacon wrote:
>>> The WnR bit in the HSR/ESR_EL2 indicates whether a data abort was
>>> generated by a read or a write instruction. For stage 2 data aborts
>>> generated by a stage 1 translation table walk (i.e. the actual page
>>> table access faults at EL2), the WnR bit therefore reports whether the
>>> instruction generating the walk was a load or a store, *not* whether the
>>> page table walker was reading or writing the entry.
>>>
>>> For page tables marked as read-only at stage 2 (e.g. due to KSM merging
>>> them with the tables from another guest), this could result in livelock,
>>> where a page table walk generated by a load instruction attempts to
>>> set the access flag in the stage 1 descriptor, but fails to trigger
>>> CoW in the host since only a read fault is reported.
>>>
>>> This patch modifies the arm64 kvm_vcpu_dabt_iswrite function to
>>> take into account stage 2 faults in stage 1 walks. Since DBM cannot be
>>> disabled at EL2 for CPUs that implement it, we assume that these faults
>>> are always causes by writes, avoiding the livelock situation at the
>>> expense of occasional, spurious CoWs.
>>>
>>> We could, in theory, do a bit better by checking the guest TCR
>>> configuration and inspecting the page table to see why the PTE faulted.
>>> However, I doubt this is measurable in practice, and the threat of
>>> livelock is real.
>>>
>>> Cc: Marc Zyngier <marc.zyngier@arm.com>
>>> Cc: Christoffer Dall <christoffer.dall@linaro.org>
>>> Cc: Julien Grall <julien.grall@arm.com>
>>> Signed-off-by: Will Deacon <will.deacon@arm.com>
>>
>> Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
>>
>> Applied,
> 
> This doesn't seem to be in 4.9-rc1. Could you please dig it up?

Looks like this patch has been lingering in -queue. I'll push it on
master as a fix for -rc2.

Thanks for the heads up.

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