Re: [PATCH 07/12] x86/virt/guest/xen: Remove use of pgd_list from the Xen guest code

From: Ingo Molnar <mingo@kernel.org>
To: David Vrabel <david.vrabel@citrix.com>
Cc: Waiman Long <Waiman.Long@hp.com>,
	Denys Vlasenko <dvlasenk@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Brian Gerst <brgerst@gmail.com>, "H. Peter Anvin" <hpa@zytor.com>,
	Oleg Nesterov <oleg@redhat.com>,
	linux-kernel@vger.kernel.org,
	Andy Lutomirski <luto@amacapital.net>,
	linux-mm@kvack.org, Borislav Petkov <bp@alien8.de>,
	Ian Campbell <ijc@hellion.org.uk>,
	xen-devel@lists.xenproject.org,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 07/12] x86/virt/guest/xen: Remove use of pgd_list from the Xen guest code
Date: Mon, 15 Jun 2015 22:35:32 +0200	[thread overview]
Message-ID: <20150615203532.GC13273__33110.3169745349$1434400682$gmane$org@gmail.com> (raw)
In-Reply-To: <557EA944.9020504@citrix.com>

* David Vrabel <david.vrabel@citrix.com> wrote:

> On 15/06/15 10:05, Ian Campbell wrote:
> > On Sat, 2015-06-13 at 11:49 +0200, Ingo Molnar wrote:

> >> xen_mm_pin_all()/unpin_all() are used to implement full guest instance 
> >> suspend/restore. It's a stop-all method that needs to iterate through all 
> >> allocated pgds in the system to fix them up for Xen's use.
> >>
> >> This code uses pgd_list, probably because it was an easy interface.
> >>
> >> But we want to remove the pgd_list, so convert the code over to walk all 
> >> tasks in the system. This is an equivalent method.
> 
> It is not equivalent because pgd_alloc() now populates entries in pgds that are 
> not visible to xen_mm_pin_all() (note how the original code adds the pgd to the 
> pgd_list in pgd_ctor() before calling pgd_prepopulate_pmd()).  These newly 
> allocated page tables won't be correctly converted on suspend/resume and the new 
> process will die after resume.

So how should the Xen logic be fixed for the new scheme? I can't say I can see 
through the paravirt complexity here.

Thanks,

	Ingo