Re: [Xen-devel] [PATCH v2 2/2] xen/efi: optionally call SetVirtualAddressMap() - Marek Marczykowski-Górecki

From: "Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Tim Deegan <tim@xen.org>, Ian Jackson <ian.jackson@eu.citrix.com>,
	Julien Grall <julien.grall@arm.com>,
	Jan Beulich <jbeulich@suse.com>,
	xen-devel@lists.xenproject.org
Subject: Re: [Xen-devel] [PATCH v2 2/2] xen/efi: optionally call SetVirtualAddressMap()
Date: Sat, 12 Oct 2019 19:09:25 +0200	[thread overview]
Message-ID: <20191012170925.GC28966@mail-itl> (raw)
In-Reply-To: <20191012170040.GB28966@mail-itl>

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On Sat, Oct 12, 2019 at 07:00:43PM +0200, Marek Marczykowski-Górecki wrote:
> On Sat, Oct 12, 2019 at 05:29:34PM +0100, Andrew Cooper wrote:
> > On 12/10/2019 15:36, Marek Marczykowski-Górecki wrote:
> > > SetVirtualAddressMap() can be called only once,
> > 
> > True.
> > 
> > > hence it's incompatible with kexec.
> > 
> > Most certainly not.
> > 
> > Linux unconditionally enters virtual mode, citing a huge slew of EFI
> > firmware bugs, and is perfectly capable of kexec-ing on the resulting
> > systems.
> > 
> > This is how Xen should behave as well, and I suspect it will have a
> > marked improvement on our ability to actually boot on EFI systems.
> > 
> > 
> > Now - it may be true that Xen is missing some piece of plumbing to allow
> > kexec in virtual mode to work, and that is a fine reason to leave a note
> > in the text of an EXPERT option noting what what is/isn't expected to
> > work (and what may or may not have been tested).
> > 
> > > For this reason, make it an optional feature, depending on
> > > !KEXEC.
> > 
> > This presupposes (at Xen's build time) that a kexec'd kernel is going to
> > want/need to use runtime services.  I'm not convinced this is
> > universally true,
> 
> In fact, as it turned out in the discussion, right now it definitely
> can't, as it doesn't get runtime services table (or efi system table or
> any other info required for this). So, it looks like it should read "it
> might be incompatible with some future Xen implementation of kexec".

Specifically, dependency on !KEXEC isn't needed right now.

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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