Re: [Qemu-devel] [patch 2/2] i386: pc: align gpa<->hpa on 1GB boundary

[Marcelo Tosatti]

On Mon, Oct 28, 2013 at 12:04:06PM -0200, Marcelo Tosatti wrote:
> On Sun, Oct 27, 2013 at 04:20:44PM +0100, igor Mammedov wrote:
> > > Yes, thought of that, unfortunately its cumbersome to add an interface
> > > for the user to supply both 2MB and 1GB hugetlbfs pages.
> > Could 2Mb tails be automated, meaning if host uses 1Gb hugepages and
> > there is/are tail/s, QEMU should be able to figure out alignment
> > issues and allocate with appropriate pages.
> 
> Yes that would be ideal but the problem with hugetlbfs is that pages are
> preallocated.
> 
> So in the end you'd have to expose the split of guest RAM in 2MB/1GB types
> to the user (it would be necessary for the user to calculate the size of
> the hole, etc).

Note the assumption here is this: its easier for the hugetlbfs user to
manage 

number of 1GB hugepages = size of guest RAM

Than for him to calculate the size of the hole (which depends on machine
type), allocate 1GB/2MB hugepages accordingly. And the benefit of that
would be to save one 1GB hugepage (which is preallocated during boot, in
the first place).

So matching number of 1GB hugepages and guest RAM seems the easier
choice.

> > Goal is separate host part allocation aspect from guest related one,
> > aliasing 32-bit hole size at the end doesn't help it at all, it's quite
> > opposite, it's making current code more complicated and harder to fix
> > in the future.

What is the problem to be fixed, exactly?

> You can simply back the 1GB areas which the hole reside with 2MB pages.
> Can't see why having the tail of RAM map to the hole is problematic.
> 
> Understand your concern, but the complication is necessary: the host
> virtual/physical address and guest physical addresses must be aligned on
> largepage boundaries.
> 
> Do you foresee any problem with memory hotplug?
> 
> Could add a warning to memory API: if memory region is larger than 1GB
> and RAM is 1GB backed, and not properly aligned, warn.