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

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Date:	Fri, 07 Sep 2012 09:06:28 +0100
From:	Matt Fleming <matt.fleming@...el.com>
To:	Jan Beulich <JBeulich@...e.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 Fri, 2012-09-07 at 08:43 +0100, Jan Beulich wrote:
> >>> 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.

Yeah, unfortunately I only realised that after I'd sent the patch.
Switching pagetables around efi calls is a much better approach.

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