linux-kernel - Re: [PATCH v4] kvm,x86: Exit to user space in case page fault error

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Message-ID: <20201006141501.GC5306@redhat.com>
Date:   Tue, 6 Oct 2020 10:15:01 -0400
From:   Vivek Goyal <vgoyal@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        virtio-fs-list <virtio-fs@...hat.com>, pbonzini@...hat.com
Subject: Re: [PATCH v4] kvm,x86: Exit to user space in case page fault error

On Tue, Oct 06, 2020 at 04:05:16PM +0200, Vitaly Kuznetsov wrote:
> Vivek Goyal <vgoyal@...hat.com> writes:
> 
> > A. Just exit to user space with -EFAULT (using kvm request) and don't
> >    wait for the accessing task to run on vcpu again. 
> 
> What if we also save the required information (RIP, GFN, ...) in the
> guest along with the APF token

Can you elaborate a bit more on this. You mean save GFN on stack before
it starts waiting for PAGE_READY event?

> so in case of -EFAULT we can just 'crash'
> the guest and the required information can easily be obtained from
> kdump? This will solve the debugging problem even for TDX/SEV-ES (if
> kdump is possible there).

Just saving additional info in guest will not help because there might
be many tasks waiting and you don't know which GFN is problematic one.

Thanks
Vivek