linux-kernel - Re: [PATCH] x86-64: use EFI to deal with platform wall clock

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Message-ID: <20120516123912.GA20990@srcf.ucam.org>
Date:	Wed, 16 May 2012 13:39:12 +0100
From:	Matthew Garrett <mjg59@...f.ucam.org>
To:	Jan Beulich <JBeulich@...e.com>
Cc:	mingo@...e.hu, tglx@...utronix.de, matt.fleming@...ux.intel.com,
	linux-kernel@...r.kernel.org, hpa@...or.com
Subject: Re: [PATCH] x86-64: use EFI to deal with platform wall clock

On Wed, May 16, 2012 at 01:18:28PM +0100, Jan Beulich wrote:

> Okay, looks like calling efi_ioremap() at this point is possible.
> But why is efi_enter_virtual_mode() being called as late as is
> the case currently for x86 anyway?

Assuming that things are as they are for a good reason is not 
necessarily true in the EFI code...

> And then again the current logic in efi_enter_virtual_mode()
> looks flawed (it assumes two contiguous pieces of direct
> mappings, and while on systems with dis-contiguous physical
> memory this currently appears to be true, it's not correct - the
> holes could have MMIO assignments in them - and hence
> shouldn't be relied upon), and I wouldn't want to copy this
> elsewhere.

Could you elaborate on that a little?

> Plus the use of set_virtual_address_map is bogus in the first
> place, as it makes it impossible for a kexec-ed kernel to also
> use EFI services (as it would have to call the function a
> second and possibly third time, yet it is not permitted to be
> called more than once). Imo all calls have to happen in
> physical mode.

Platforms don't correctly deal with the case where you make physical 
calls after ExitBootServices(). We tried running in physical mode. It 
simply doesn't work.

> So I'm afraid if the patch as I provided it isn't acceptable, and
> if the call to efi_enter_virtual_mode() can't be moved ahead
> of the one to timekeeping_init(), this winds down to the whole
> logic needing a re-write.

I have zero objection to this being cleaned up, but I don't know of any 
obvious reason why we can't do enter_virtual_mode() earlier.

-- 
Matthew Garrett | mjg59@...f.ucam.org
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