Re: Reducing or removing direct map from xen (was Re: Ongoing/future speculative mitigation work)

From: Wei Liu <wei.liu2@citrix.com>
To: "Roger Pau Monné" <roger.pau@citrix.com>
Cc: "Martin Pohlack" <mpohlack@amazon.de>,
	"Julien Grall" <julien.grall@arm.com>,
	"Jan Beulich" <JBeulich@suse.com>,
	"Joao Martins" <joao.m.martins@oracle.com>,
	"Stefano Stabellini" <sstabellini@kernel.org>,
	"Daniel Kiper" <daniel.kiper@oracle.com>,
	"Marek Marczykowski" <marmarek@invisiblethingslab.com>,
	"Anthony Liguori" <aliguori@amazon.com>,
	Xen-devel <xen-devel@lists.xenproject.org>,
	"Dannowski, Uwe" <uwed@amazon.de>,
	"Lars Kurth" <lars.kurth@citrix.com>,
	"Konrad Wilk" <konrad.wilk@oracle.com>,
	"Ross Philipson" <ross.philipson@oracle.com>,
	"Dario Faggioli" <dfaggioli@suse.com>,
	"Matt Wilson" <msw@amazon.com>,
	"Boris Ostrovsky" <boris.ostrovsky@oracle.com>,
	"Juergen Gross" <JGross@suse.com>,
	"Sergey Dyasli" <sergey.dyasli@citrix.com>,
	"Wei Liu" <wei.liu2@citrix.com>,
	"George Dunlap" <george.dunlap@eu.citrix.com>,
	"Andrew Cooper" <andrew.cooper3@citrix.com>,
	"Mihai Donțu" <mdontu@bitdefender.com>
Subject: Re: Reducing or removing direct map from xen (was Re: Ongoing/future speculative mitigation work)
Date: Thu, 21 Feb 2019 17:51:00 +0000	[thread overview]
Message-ID: <20190221175100.tbfmagdo74vaal5f@zion.uk.xensource.com> (raw)
In-Reply-To: <20190221095941.6ssrgwiugcr7vtb3@Air-de-Roger>

On Thu, Feb 21, 2019 at 10:59:41AM +0100, Roger Pau Monné wrote:
> On Wed, Feb 20, 2019 at 05:08:09PM +0000, Wei Liu wrote:
> > On Wed, Feb 20, 2019 at 01:09:56PM +0000, Wei Liu wrote:
> > [...]
> > > I think under-allocate-then-map looks plausible. xmalloc will need
> > > to allocate pages, put them into an array and call __vmap on that array
> > > directly.
> > 
> > The biggest issue with this approach is that we now need an array of
> > 1UL<<MAX_ORDER to accommodate mfns. Back of envelope calculation: on x86
> > this is going to be (1UL<<20)*8 bytes long. This is not feasible.
> 
> Right. I guess the only remaining option is to allocate a virtual
> address space and populate it using multiple pages?

I probably was not clear enough -- the aforementioned array was needed
for this method, so I don't think this is feasible.

> 
> That would likely require to split some functions into smaller helpers
> so you can call them and provide the virtual address where a page
> should be mapped?
> 

Not feasible as of now. The fundamental issue is that vmap is managed by
a bitmap which has 1:1 mapping for page and address space, and it
mandates a guard page for each allocation.

To make this work we need to remove the mandatory guard page. Inventing
more complex tracking structure is probably not worth it.

Wei.

> Roger.

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