linux-kernel - Re: [RFC RESEND PATCH] kvm: arm64: export memory error recovery capability to user space

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Date:   Fri, 14 Dec 2018 14:33:53 +0000
From:   Peter Maydell <peter.maydell@...aro.org>
To:     James Morse <james.morse@....com>
Cc:     gengdongjiu <gengdongjiu@...wei.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Christoffer Dall <christoffer.dall@....com>,
        Marc Zyngier <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        kvm-devel <kvm@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
        arm-mail-list <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC RESEND PATCH] kvm: arm64: export memory error recovery
 capability to user space

On Fri, 14 Dec 2018 at 13:56, James Morse <james.morse@....com> wrote:
>
> Hi Dongjiu Geng,
>
> On 14/12/2018 10:15, Dongjiu Geng wrote:
> > When user space do memory recovery, it will check whether KVM and
> > guest support the error recovery, only when both of them support,
> > user space will do the error recovery. This patch exports this
> > capability of KVM to user space.
>
> I can understand user-space only wanting to do the work if host and guest
> support the feature. But 'error recovery' isn't a KVM feature, its a Linux
> kernel feature.
>
> KVM will send it's user-space a SIGBUS with MCEERR code whenever its trying to
> map a page at stage2 that the kernel-mm code refuses this because its poisoned.
> (e.g. check_user_page_hwpoison(), get_user_pages() returns -EHWPOISON)
>
> This is exactly the same as happens to a normal user-space process.
>
> I think you really want to know if the host kernel was built with
> CONFIG_MEMORY_FAILURE.

Does userspace need to care about that? Presumably if the host kernel
wasn't built with that support then it will simply never deliver
any memory failure events to QEMU, which is fine.

The point I was trying to make in the email Dongjiu references
(https://patchwork.codeaurora.org/patch/652261/) is simply that
"QEMU gets memory-failure notifications from the host kernel"
does not imply "the guest is prepared to receive memory
failure notifications", and so the code path which handles
the SIGBUS must do some kind of check for whether the guest
CPU is a type which expects them and that the board code
set up the ACPI tables that it wants to fill in.

thanks
-- PMM