Re: [Xen-devel] Re: [PATCH 00 of 36] x86/paravirt: groundwork for 64-bit Xen support

Keir Fraser
2008-06-25T19:23:18Z

On 25/6/08 20:13, "Andi Kleen" <andi@firstfloor.org> wrote:

>> What does Linux expect to scale up to? Reserving 16 PML4 entries leaves the
>> kernel with 120TB of available 'negative' address space. Should be plenty, I
>> would think.
>
> There are already (ok non x86-64) systems shipping today with 10+TB of
> addressable memory. 100+TB is not that far away with typical
> growth rates. Besides there has to be much more in the negative address
> space than just direct mapping.

There are obviously no x64 boxes around at the moment with >1TB of regular
shared memory, since no CPUs have more than 40 address lines. 100+TB RAM is
surely years away.

If this is a blocker issue, we could just keep PAGE_OFFSET as it is when Xen
support is not configured into the kernel. Then those who are concerned
about 5% extra headroom at 100TB RAM sizes can configure their kernel
appropriately.

> So far we always that 64bit Linux can support upto 1/4*max VA memory.
> With your change that formula would be not true anymore.

Does the formula have any practical significance?

-- Keir