linux-kernel - Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request at 0000a7cf

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Date:   Thu, 9 Mar 2017 15:49:24 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Daniel Borkmann <daniel@...earbox.net>
cc:     Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>, Peter Anvin <hpa@...or.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, LKP <lkp@...org>,
        ast@...com, the arch/x86 maintainers <x86@...nel.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request
 at 0000a7cf

On Thu, 9 Mar 2017, Daniel Borkmann wrote:

> On 03/09/2017 02:10 PM, Thomas Gleixner wrote:
> > On Thu, 9 Mar 2017, Daniel Borkmann wrote:
> > > With regard to CPA_FLUSHTLB that Linus mentioned, when I investigated
> > > code paths in change_page_attr_set_clr(), I did see that CPA_FLUSHTLB
> > > was set each time we switched attrs and a cpa_flush_range() was
> > > performed (with the correct number of pages and cache set to 0). That
> > > would be a __flush_tlb_all() eventually.
> > > 
> > > Hmm, it indeed might seem likely that this could be an emulation bug.
> > 
> > Which variant of __flush_tlb_all() is used when the test fails?
> > 
> > Check for the following flags in /proc/cpuinfo: pge invpcid
> 
> I added the following and booted with both variants:
> 
> printk("X86_FEATURE_PGE:%u\n",     static_cpu_has(X86_FEATURE_PGE));
> printk("X86_FEATURE_INVPCID:%u\n", static_cpu_has(X86_FEATURE_INVPCID));
> 
> "-cpu host" gives:
> 
> [    8.326117] X86_FEATURE_PGE:1
> [    8.326381] X86_FEATURE_INVPCID:1
> 
> "-cpu kvm64" gives:
> 
> [    8.517069] X86_FEATURE_PGE:1
> [    8.517393] X86_FEATURE_INVPCID:0

That's the one which fails. So it's using the CR4 based flushing. Just ran
a test on a physical system with PGE=1 and INVPCID=0. Works fine.

Emulation problem?

Thanks,

	tglx