linux-kernel - Re: [PATCH] x86, efi: 1:1 pagetable mapping for virtual EFI calls

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Date:	Fri, 07 Sep 2012 08:43:55 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Matt Fleming" <matt.fleming@...el.com>
Cc:	"Vasco Dias" <rafa.vasco@...il.com>,
	"Matthew Garrett" <mjg@...hat.com>, <linux-efi@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <cJ-ko@...gloub.eu>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86, efi: 1:1 pagetable mapping for virtual EFI
 calls

>>> On 06.09.12 at 17:47, Matt Fleming <matt.fleming@...el.com> wrote:
> On Thu, 2012-09-06 at 15:34 +0100, Jan Beulich wrote:
>> >>> On 06.09.12 at 15:15, Matt Fleming <matt@...sole-pimps.org> wrote:
>> > +
>> > +		pgd += i;
>> > +		save[i] = *pgd;
>> > +		set_pgd(pgd, efi_one_to_one_pgd[i]);
>> > +	}
>> 
>> Did you, as an alternative, consider switching to a different
>> CR3 instead of copying back and forth?
> 
> I did consider it, but I couldn't convince myself whether or not the EFI
> pagetable would need to be manually kept in sync with any other
> pagetables. But now I look at the code a bit harder, it seems that
> should be taken care of automatically. In fact, the tboot code seems to
> do something similar. I'll try that approach.

Actually, I think the copying approach is even broken - what if
multiple threads currently on the same address space want to
invoke a runtime call simultaneously? The first one to get here
would save the right values, but the second one wouldn't.

Jan

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