Re: [PATCH 1/4] x86/mm/pat: Handle no-GBPAGES case correctly in populate_pud

[Arvind Sankar <nivedita@alum.mit.edu>]

On Wed, Mar 04, 2020 at 08:04:04PM +0100, Ard Biesheuvel wrote:
> On Wed, 4 Mar 2020 at 19:50, Arvind Sankar <nivedita@alum.mit.edu> wrote:
> >
> > On Wed, Mar 04, 2020 at 07:44:50PM +0100, Ard Biesheuvel wrote:
> > >
> > > I've tried a couple of different ways, but I can't seem to get my
> > > memory map organized in the way that will trigger the error.
> >
> > What does yours look like? efi_merge_regions doesn't merge everything
> > that will eventually be mapped the same way, so if there are some
> > non-conventional memory regions scattered over the address space, it
> > might be breaking up the mappings to the point where this doesn't
> > trigger.
> 
> I have a region
> 
> [    0.000000] efi: mem07: [Conventional Memory|   |  |  |  |  |  |  |
>  |   |WB|WT|WC|UC] range=[0x0000000001400000-0x00000000b9855fff]
> (2948MB)
> 
> which gets covered correctly
> 
> [    0.401766] 0x0000000000a00000-0x0000000040000000        1014M
> RW         PSE         NX pmd
> [    0.403436] 0x0000000040000000-0x0000000080000000           1G
> RW         PSE         NX pud
> [    0.404645] 0x0000000080000000-0x00000000b9800000         920M
> RW         PSE         NX pmd
> [    0.405844] 0x00000000b9800000-0x00000000b9a00000           2M
> RW                     NX pte
> [    0.407436] 0x00000000b9a00000-0x00000000baa00000          16M
> ro         PSE         x  pmd
> [    0.408591] 0x00000000baa00000-0x00000000bbe00000          20M
> RW         PSE         NX pmd
> [    0.409751] 0x00000000bbe00000-0x00000000bc000000           2M
> RW                     NX pte
> [    0.410821] 0x00000000bc000000-0x00000000be600000          38M
> RW         PSE         NX pmd
> 
> However, the fact that you can provide a case where it does fail
> should be sufficient justification for taking this patch. I was just
> trying to give more than a regression-tested-by

No, this case is exactly one that should break. But I think you're
running on a processor model that _does_ support GB pages, as shown by
the "pud" mapping there for the 1G-2G range.

At least for my version of qemu, -cpu Haswell does not enable the
pdpe1gb feature. Which cpu did you specify?

Thanks.