linux-kernel - Re: Random crashes with i386 and efi boots

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Message-ID: <20180911175940.GB15367@roeck-us.net>
Date:   Tue, 11 Sep 2018 10:59:40 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Joerg Roedel <jroedel@...e.de>
Cc:     Andy Lutomirski <luto@...capital.net>,
        linux-kernel@...r.kernel.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Michal Hocko <mhocko@...e.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Pavel Machek <pavel@....cz>, linux-efi@...r.kernel.org,
        x86@...nel.org
Subject: Re: Random crashes with i386 and efi boots

On Tue, Sep 11, 2018 at 07:41:58PM +0200, Joerg Roedel wrote:
> On Tue, Sep 11, 2018 at 09:36:51AM -0700, Andy Lutomirski wrote:
> > >    save_pgd = efi_call_phys_prolog();
> > >    local_irq_save(flags);
> > >    status = efi_call_phys(...);
> > >        local_irq_restore(flags);
> > > 
> > >        efi_call_phys_epilog(save_pgd);
> > > 
> > > So, yes, interrupts are very much enabled.
> > 
> > Does fixing that solve the problem?  It seems more robust.
> 
> The problem is still that in efi_call_phys_prolog() we load the gdt with
> its physical address, and when we reload the %cr3 in _epilog from
> initial_page_table to swapper_pg_dir again the gdt is no longer mapped.
> Blocking interrupts is more robust, but we can't block NMIs that way
> that would also trigger the issue, no?
> 
> So I am in favor of changing the order in efi_call_phys_epilog() too.
> 
Blocking the interrupts across efi_call_phys_prolog() / efi_call_phys() /
efi_call_phys_epilog() does solve the issue, but I am concerned about its
negative effect on 64-bit operation (not sure if blocking interrupts
in the prolog/epilog functions there is ok). Adding another set of
local_irq_save() / local_irq_restore() to efi_call_phys_epilog() seems
wasteful if the problem can be solved by reordering instructions.

So, yes, I also prefer changing the order in efi_call_phys_epilog().
I'll submit a patch along that line.

Thanks,
Guenter