Re: [PATCH 4/4] target/arm: Retry pushing CPER error if necessary

[Gavin Shan <gshan@redhat.com>]

On 2/21/25 9:04 PM, Jonathan Cameron wrote:
> On Fri, 21 Feb 2025 15:27:36 +1000
> Gavin Shan <gshan@redhat.com> wrote:

[...]
   
>>
>> I would say #1 is the ideal model because the read_ack_register is the bottleneck
>> and it should be scaled up to max_cpus. In that way, the bottleneck can be avoided
>> from the bottom. Another benefit with #1 is the error can be delivered immediately
>> to the vCPU where the error was raised. This matches with the syntax of SEA to me.
> 
> I don't think it helps for the bottleneck in linux at least.  A whole bunch of locks
> are taken on each SEA because of the novel use of the fixmap.  There is only one
> VA ever used to access the error status blocks we just change what PA it points to
> under a spin lock. Maybe that can be improved on if we can persuade people that error
> handling performance is a thing to care about!
> 

Right, it doesn't helps for the bottleneck in guest kernel due to @ghes_notify_lock_sea.
With the lock, all existing GHES devices and error statuses are serialized for access. I
was actually talking about the benefit to avoid the bottleneck regarding the read_ack_regsiter,
which is the synchronization mechanism between guest kernel and QEMU. For example, an error
has been raised on vCPU-0, but not acknowledged at (A). Another error raised on vCPU-1
can't be delivered because we have only one GHES device and error status block, which
has been reserved for the error raised on vCPU-0.  With solution #1, the bottleneck can
be avoided with multiple GHES devices and error status blocks.

   vCPU-0                                           vCPU-1
   ======                                           ======
   kvm_cpu_exec                                     kvm_cpu_exec
     kvm_vcpu_ioctl(RUN)                              kvm_vcpu_ioctl(RUN)
     kvm_arch_on_sigbus_vcpu                          kvm_arch_on_sigbus_vcpu
       acpi_ghes_memory_errors                          acpi_ghes_memory_errors   (B)
       kvm_inject_arm_sea
     kvm_vcpu_ioctl(RUN)
       :
     do_mem_abort
       do_sea
         apei_claim_sea
           ghes_notify_sea
             raw_spin_lock(&ghes_notify_lock_sea)
             ghes_in_nmi_spool_from_list
               ghes_in_nmi_queue_one_entry
                 ghes_clear_estatus                 (A)
             raw_spin_unlock(&ghes_notify_lock_sea)

Thanks,
Gavin