Re: [PATCH v2] KVM: arm/arm64: Signal SIGBUS when stage2 discovers hwpoison memory

James Morse
2017-06-22T16:35:47Z

Hi gengdongjiu,

On 22/06/17 07:47, gengdongjiu wrote:
> On 2017/6/21 20:44, James Morse wrote:
>> On 21/06/17 11:59, gengdongjiu wrote:
>>> On 2017/6/21 17:53, James Morse wrote:
>>>> I think we discussed this before[0], your CPU has a feature called 'hwpoison'
>>>> that is uses to support RAS. Linux also has a feature called 'hwpoison' [1][2],
>>>> which handles the offline-ing of memory pages when it receives a notification
>>>> through APEI. I've tried to call this memory_failure() to avoid this confusion.
>>>>
>>>> This patch is to handle stage2 faults when the page was removed from the stage2
>>>> mapping by the memory_failure() code. v3 of this patch[3] does a much better job
>>>> of describing this.
>>>>
>>>> (... I don't think your question is related to this patch ...)
>>>
>>> I know your meaning about the Linux 'hwpoison' feature.

>> Okay, I assume we are also talking about firmware-first RAS events and your APEI
>> notifications use SEA.

It doesn't look like you are using APEI firmware-first or SEA notifications here.

>> I think we are looking at different parts of the code, here is what I see should
>> happen:
>>
>> For a Synchronous External Abort the ESR {D,I}FSC bits will be in the range that
>> indicates an external abort. For a data:external-abort kvm_handle_guest_abort()
>> will matches this with kvm_vcpu_dabt_isextabt() and makes no further attempt to
>> handle the fault.

> The kvm_vcpu_dabt_isextabt() will check the ESR EA bit[9]
> The EA bit [9] is "IMPLEMENTATION DEFINED", so whether match with the kvm_vcpu_dabt_isextabt() depend on the CPU design.
> In my platform the EA bit is zero for the RAS Synchronous External Abort, so will not matches the kvm_vcpu_dabt_isextabt(),
> so the code will continue handle the fault.

This is the problem, you aren't notifying RAS errors to the OS with an
SEA/synchronous-external-abort.

For APEI firmware-first RAS you must use one of the notification methods listed
in ACPI 6.2s '18.3.2.9 Hardware Error Notification, Table 18-383'.

Notifications using 'SEA' hang on an existing part of the ARM architecture,
firmware has to emulate what the CPU does to avoid breaking existing code.

> EA, bit [9]
> External Abort. As described in the Architecture Reference Manual. Provides an IMPLEMENTATION
> DEFINED classification of the external abort.

This is from 'D7.2.28 - ESR_ELx, Exception Syndrome Register (ELx)' of
DDI0487B.a. If you don't set that bit, its not an external abort.
Implementation-defined refers to the classification: your CPU implementation is
allowed to choose what it classifies as 'external'.
For RAS, your firmware must set this bit if it wants to use SEA as the APEI
notification type.

What is the ESR_EL2 value when this happens?
Do you trap this to firmware first? (its not firmware-first RAS if you don't).

>> This goes on to call ghes_notify_sea() which will handle the error and cause KVM
>> to exit this function. KVM makes no further attempt to handle the fault as APEI
>> should have done everything necessary. KVM will re-enter the guest, unless there
>> are signals pending.

> In my code, there is not Tyler's such modification.

Tyler's series added support to KVM for handling RAS notifications via SEA.

> firstly, the poison error is related with RAS, and poison error match the is_abort_sea(fault_status)

(So it is an external abort? Now I'm confused)

> so the KVM will exit, and SIGBUS signal have no chance to be delivered.

kvm_handle_guest_abort() will return 1 through handle_exit() and into
kvm_arch_vcpu_ioctl_run()'s while (ret > 0) { } loop.

KVM should re-enter the guest because the APEI GHES RAS code said it had handled
the error.

> I suggest it does not return in Tyler's patch

This is required because the notification about a RAS fault may not be a
sensible guest fault. In your example its not a sensible guest exit because you
haven't set HPFAR_EL2. KVM can't handle a guest fault without this register.

> or move kvm_send_hwpoison_signal function before return.

Here we are talking about RAS notifications from firmware. (1)

kvm_send_hwpoison_signal() is for handling normal stage2 faults where the page
to map has been poisoned (2). By this point the CPU's RAS features and firmware
are not involved.

These are two different things, a well behaved machine-monitor and guest will
never touch a poisoned page once its received a notification due to (1). This
patch addresses a corner case that covers Qemu/kvmtool not handling (1)
properly, or the guest ignoring the notification, (and all the races in between).

>> (You're right that here the fault_ipa is the wrong thing to pass, but
>> handle_guest_sea() doesn't use it...)
>>
>> We do need to enable HCR_EL2.TEA which was added with v8.2s RAS extensions, but
>> the cpufeature patch is a pre-requisite.

> HCR_EL2.TEA will Route synchronous External Abort exceptions from Non-secure EL0 and EL1 to EL2, if not routed to EL3.
>
> in the firmware-first RAS solution, this bit can not control to route to El2 because the exception is route to EL3.
>
> enabling HCR_EL2.TEA can be better for the non firmware-first RAS solution.
> but can not solve this issue.

Your firmware receives an external abort from the CPU, trapped to EL3 by
SCR_EL3.EA. It chooses to notify the OS via APEI's SEA notification type. How
does it choose whether to target EL2 or EL1 for the fake software-generated
SEA-notification? To correctly emulate what the CPU would do, it should inspect
HCR_EL2.TEA, and route the notification accordingly.

> my firmware will route the injected-External-Aborts to the hypervisor if the
> error come from guest OS.

How do you know there is a hypervisor at EL2 and guest OS at EL1? Disable both
KVM and VHE in Linux and boot. Now you have host Linux at EL1 and only the
hyp-stub at EL2.

These are the complications with hanging APEI notification on an existing part
of the architecture.

>> The path to the below code that uses fault_ipa->gfn->pfn->hva is:
>> kvm_handle_guest_abort()
>> user_mem_abort()
>> kvm_send_hwpoison_signal().
>>
>> But for an external abort due to RAS we never get past kvm_handle_guest_abort()
>> as we call out to the APEI ghes to handle the RAS error instead.

> so this patch can not work.

To notify Qemu/kvmtool about a RAS fault? That's not what this patch does.

v3 of this patch has a much better commit message:
----%<----
memory_failure() has two modes, early and late. Early is used by
machine-managers like Qemu to receive a notification when a memory error is
notified to the host. These can then be relayed to the guest before the
affected page is accessed. To enable this, the process must set
PR_MCE_KILL_EARLY in PR_MCE_KILL_SET using the prctl() syscall.

Once the early notification has been handled, nothing stops the
machine-manager or guest from accessing the affected page. If the
machine-manager does this the page will fail to be mapped and SIGBUS will
be sent. This patch adds the equivalent path for when the guest accesses
the page, sending SIGBUS to the machine-manager.

These two signals can be distinguished by the machine-manager using their
si_code: BUS_MCEERR_AO for 'action optional' early notifications, and
BUS_MCEERR_AR for 'action required' synchronous/late notifications.
----%<----

On receipt of a RAS notification, KVM calls out to the APEI code which handles
the error. KVM can now go back to running the guest.

Handling the error may have caused a signal to be sent to Qemu, in which case we
won't get back into the guest, as the signal_pending() test in
kvm_arch_vcpu_ioctl_run() will cause us to return to Qemu instead. This is the
early path. It happens when we get the RAS notification from firmware.

This patch is concerned with the late path. When either Qemu/kvmtool or the
guest failed to handle the early notification and went and touched the affected
memory again. In this case we don't take a fault via firmware as
memory_failure() has unmapped the page from stage2, we take a normal stage2
fault instead. KVM won't re-map the poisoned page, it sends a late notification
instead.

Thanks,

James