linux-kernel - Re: [PATCH] phys_efi_set_virtual_address

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Date:	Thu, 21 Jun 2012 15:16:06 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>
Cc:	Robin Holt <holt@....com>, Matthew Garrett <mjg59@...f.ucam.org>,
	linux-kernel@...r.kernel.org,
	"Sakkinen, Jarkko" <jarkko.sakkinen@...el.com>
Subject: Re: [PATCH] phys_efi_set_virtual_address_map needs va, no pa.

On Wed, Jun 20, 2012 at 05:46:49PM -0700, H. Peter Anvin wrote:
> On 06/20/2012 05:27 PM, Robin Holt wrote:
> > 
> > What do you need from me?  If you want me to help with this, I have a
> > _WHOLE_ lot of learning to do.  Can you give me any pointers?
> > 
> > We are trying to get this finally fixed.  We have had work-around code
> > in SLES11 SP1, SLES11 SP2, and RHEL 6.x.  I would love to get this fixed
> > for future distro snaps.
> > 
> 
> If you want to tackle it, the task is basically that when we modify the
> pgds in 32-bit legacy (non-PAE) mode, we should make the corresponding
> modifications to initial_page_table, and in 64-bit mode to
> real_mode_header->trampoline_pgd.  It might be worthwhile to introduce a
> common pointer for both, obviously.
> 
> This is currently handled via something called the pgd_list (when we
> update the top level kernel address space we walk pgd_list and update
> them all), but there are two issues:
> 
> 1. Obviously, in the case of the 1:1 map, we don't just need to maintain
> the kernel area, but the "user space" part of the address space should
> contain a copy, as well.
> 
> 2. To complicate things, there is code in there to grab an mm lock for
> the benefit of Xen.  The 1:1 map doesn't have an mm associated with it,
> so I'm not quite sure how that is to be handled.  Perhaps Xen just plain
> won't need it and we can just bypass it, but I have no bloody idea.

You mean this?

79e53d8        (Andrea Arcangeli       2011-02-16 15:45:22 -0800       127)    spin_lock(&pgd_lock);
4f76cd38        (Jeremy Fitzhardinge    2008-03-17 16:36:55 -0700       128)    pgd_list_del(pgd);
a79e53d8        (Andrea Arcangeli       2011-02-16 15:45:22 -0800       129)    spin_unlock(&pgd_lock);

which says:
    x86/mm: Fix pgd_lock deadlock
    
    It's forbidden to take the page_table_lock with the irq disabled
    or if there's contention the IPIs (for tlb flushes) sent with
    the page_table_lock held will never run leading to a deadlock.
    
    Nobody takes the pgd_lock from irq context so the _irqsave can be
    removed.
    
Looking before that git commit I see Jeremy's 4f76cd38 unification of
the 32-bit and 64-bit pgtable, and before that:

1da177e4c3f41524e886b7f1b8a0c1fc7321cac2
Linux-2.6.12-rc2

I am not really convienced that lock was put there for Xen as the
git history seems to point to well, way ancient stuff.

Or are you referring to something else?

> 
> It is also a bit "cute" how we seem to make a function call to indirect
> through a pointer (why on Earth is pgd_page_get_mm() not an inline?!),
> and then grab a lock unconditionally, regardless of if we are affected
> by Xen or not.
> 
> 	-hpa
> --
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