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Re: [Xen-devel] mapping of memory below 16Mb

  • To: Jan Beulich <jbeulich@xxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
  • Date: Thu, 24 Jan 2008 16:22:40 +0000
  • Delivery-date: Thu, 24 Jan 2008 08:23:23 -0800
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: AchenYL0wYC7+8qQEdyCrwAX8io7RQAB9Pq2
  • Thread-topic: [Xen-devel] mapping of memory below 16Mb

On 24/1/08 15:26, "Keir Fraser" <Keir.Fraser@xxxxxxxxxxxx> wrote:

>> One point I got a little surprised about while doing the 1Gb page support:
>> The 1:1 mapping of the low 16 Mb (and most significantly the first Mb)
>> created at boot time is being retained permanently. Shouldn't Xen be
>> following the E820 map also for this memory range (by either not
>> mapping non-RAM at all or at best mapping them UC/WP)?
> Below 1MB is a special case, and 1MB-16MB we're dodging BIOS tables, not
> actual hardware. At least this is the case for all system I know about. If
> there are exceptions which we care about, then we can try harder. Otherwise
> that code is quite complicated enough as it is.

Actually this isn't hard to fix now I think about it. Also we should be
mapping the range 0-4MB or 0-2MB with 4kB mappings anyway, to avoid
conflicting with memory access constraints specified by the fixed MTRRs.
Every modern BIOS sets the fixed MTRRs with a mixture of memory types, and
if we map that range with a superpage we either get undefined behaviour or
at best (e.g., Intel CPUs) the processor will implicitly shatter the
superpage mapping so that we get 4kB-mapping levels of performance anyway.

This has a side 'benefit' that the range 0-1GB cannot be mapped with a 1GB
superpage mapping, and that simplifies your patch a bit (since we will never
need to free the statically-allocated first few l2e ident mapping pages).

 -- Keir

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