linux-kernel - Re: [PATCH] efi/libstub/fdt: Standardize the names of EFI stub parameters

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Date:	Fri, 11 Sep 2015 17:36:05 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Jan Beulich <JBeulich@...e.com>
Cc:	"Ian.Campbell@...rix.com" <Ian.Campbell@...rix.com>,
	"julien.grall@...rix.com" <julien.grall@...rix.com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	"freebsd-arm@...ebsd.org" <freebsd-arm@...ebsd.org>,
	"peter.huangpeng@...wei.com" <peter.huangpeng@...wei.com>,
	Shannon Zhao <zhaoshenglong@...wei.com>,
	"matt.fleming@...el.com" <matt.fleming@...el.com>,
	"ard.biesheuvel@...aro.org" <ard.biesheuvel@...aro.org>,
	"christoffer.dall@...aro.org" <christoffer.dall@...aro.org>,
	"leif.lindholm@...aro.org" <leif.lindholm@...aro.org>,
	"shannon.zhao@...aro.org" <shannon.zhao@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"daniel.kiper@...cle.com" <daniel.kiper@...cle.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] efi/libstub/fdt: Standardize the names of EFI stub
 parameters

> >> Considering that the EFI support is just for Dom0, and Dom0 (at
> >> the time) had to be PV anyway, it was the more natural solution to
> >> expose the interface via hypercalls, the more that this allows better
> >> control over what is and primarily what is not being exposed to
> >> Dom0. With the wrapper approach we'd be back to the same
> >> problem (discussed elsewhere) of which EFI version to surface: The
> >> host one would impose potentially missing extensions, while the
> >> most recent hypervisor known one might imply hiding valuable
> >> information from Dom0. Plus there are incompatible changes like
> >> the altered meaning of EFI_MEMORY_WP in 2.5.
> > 
> > I'm not sure I follow how hypercalls solve any impedance mismatch here;
> > you're still expecting Dom0 to call up to Xen in order to perform calls,
> > and all I suggested was a different location for those hypercalls.
> > 
> > If Xen is happy to make such calls blindly, why does it matter if the
> > hypercall was in the kernel binary or an external shim?
> 
> Because there could be new entries in SystemTable->RuntimeServices
> (expected and blindly but validly called by the kernel). Even worse
> (because likely harder to deal with) would be new fields in other
> structures.

Any of these could cause Xen to blow up, while Xen could always provide
a known-safe (but potentially sub-optimal) view to the kernel by
default.

> > Incompatible changes are a spec problem regardless of how this is
> > handled.
> 
> Not necessarily - we don't expose the memory map (we'd have to
> if we were to mimic EFI for Dom0), and hence the mentioned issue
> doesn't exist in our model.

We have to expose _some_ memory map, so I don't follow this point.

Mark.
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