Re: [Qemu-devel] [PATCH RESEND v15 08/10] target-arm: kvm64: inject synchronous External Abort

["Alex Bennée" <alex.bennee@linaro.org>]

Peter Maydell <peter.maydell@linaro.org> writes:

> On Wed, 21 Nov 2018 at 14:34, gengdongjiu <gengdongjiu@huawei.com> wrote:
>>
>> Hi Peter,
>>   Thanks for the review and comments.
>>
>> >
>> > On 8 November 2018 at 10:29, Dongjiu Geng <gengdongjiu@huawei.com> wrote:
>> > > +bool write_part_cpustate_to_list(ARMCPU *cpu, ptrdiff_t fieldoffset)
>> >
>> > What is this about? Nothing else in QEMU needs to mess with the cpustate synchronization. My first assumption is that you should not
>> > need to do so either.
>>
>> We should change the guest CP15 ESR_EL1's value, the only method is to change the cpu->cpreg_values[] in QEMU, then QEMU call write_list_to_kvmstate()
>> to set the cpu->cpreg_values[] to KVM which include the specified ESR_EL1 value, KVM do world switch, and then set the specified ESR_EL1's value to guest kernel.
>
> Ah, I see. This is a bug in our current handling of the register
> state, where we implicitly assume that nothing in QEMU will ever
> want to change any system register values. This assumption is
> now false -- kvm_arm_handle_debug() broke it -- so we need to
> fix the code that does kvm_arch_put_registers(). There is a comment
> in the kvm32.c version of that function about this. (The kvm64.c
> version has the same assumption but doesn't comment on it.)
>
> We should (ideally) fix this bug in the code that does register
> syncing, without requiring places in QEMU that update system
> registers to have to manually indicate which registers they have
> changed. I'll have a think about how best to do this.
>
>> About the detailed explanation, as shown in [2].
>>
>> kvm_arm_handle_debug() does not need to do this because QEMU does not need to change CP15 registers, such as ESR_EL1.
>
> kvm_arm_handle_debug does change ESR_EL1: it is injecting an exception
> and so should set the exception register. This happens when it
> calls the do_interrupt() hook, because arm_cpu_do_interrupt_aarch64()
> writes to env->cp15.esr_el[new_el].
>
> I'm not entirely sure why this is working today, in fact.
> Alex, did you test whether our debug-exception-injection
> reports the correct ESR_EL1 to the guest ?
<snip>

I did not - I was mostly focusing in the host-debugging-the-guest test
case. I'll get a test rig up and check.

--
Alex Bennée