Re: [Qemu-devel] [RFC PATCH] pc: align gpa<->hpa on 1GB boundary by splitting RAM on several regions

[Marcelo Tosatti]

On Wed, Oct 30, 2013 at 08:56:17PM +0100, Igor Mammedov wrote:
> On Wed, 30 Oct 2013 16:51:29 -0200
> Marcelo Tosatti <address@hidden> wrote:
> 
> > On Wed, Oct 30, 2013 at 05:49:49PM +0100, Igor Mammedov wrote:
> > > On Tue, 29 Oct 2013 19:38:44 -0200
> > > Marcelo Tosatti <address@hidden> wrote:
> > > 
> > > > On Tue, Oct 29, 2013 at 07:18:49PM +0100, Igor Mammedov wrote:
> > > > > Otherwise 1GB TLBs cannot be cached for the range.
> > > > 
> > > > This fails to back non-1GB-aligned gpas, but 2MB aligned, with 2MB large
> > > > pages.
> > > With current command line only one hugetlbfs mount point is possible, so 
> > > it
> > > will back with whatever alignment specified hugetlbfs mount point has.
> > > Anything that doesn't fit into page aligned region goes to tail using
> > > non hugepage baked phys_mem_set_alloc()=qemu_anon_ram_alloc() allocator.
> > 
> > The patch you propose allocates the non-1GB aligned tail of RAM with 4k
> > pages. As mentioned, this is not acceptable (2MB pages should be used
> > whenever 1GB alignment is not possible).
> > 
> > I believe its easier for the user to allocate enough 1GB pages to back
> > all of guest RAM, since allocation is static, than for him to allocate
> > mixed 1GB/2MB pages in hugetlbfs.
> tail, if it's present at all, is always less than 1Gb.
> why tail allocated with 4K pages is not acceptable?

Depends on workload.

> btw: now if QEMU can't allocate hugepages for whole guest size it will 
> fallback
> to 4k pages anyway for whole guest size, with warning that isn't visible if
> user doesn't start QEMU manually.

Not with -mem-prealloc.