Re: [Xen-devel] [PATCH v9 07/38] x86/mm: Remove phys_to_virt() usage in ioremap()

[Tom Lendacky <thomas.lendacky@xxxxxxx>]

On 7/10/2017 11:58 PM, Brian Gerst wrote:
> On Mon, Jul 10, 2017 at 3:50 PM, Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:
> > On 7/8/2017 7:57 AM, Brian Gerst wrote:
> > > On Fri, Jul 7, 2017 at 9:39 AM, Tom Lendacky <thomas.lendacky@xxxxxxx>
> > > wrote:
> > > > Currently there is a check if the address being mapped is in the ISA
> > > > range (is_ISA_range()), and if it is, then phys_to_virt() is used to
> > > > perform the mapping. When SME is active, the default is to add pagetable
> > > > mappings with the encryption bit set unless specifically overridden. The
> > > > resulting pagetable mapping from phys_to_virt() will result in a mapping
> > > > that has the encryption bit set. With SME, the use of ioremap() is
> > > > intended to generate pagetable mappings that do not have the encryption
> > > > bit set through the use of the PAGE_KERNEL_IO protection value.
> > > >
> > > > Rather than special case the SME scenario, remove the ISA range check and
> > > > usage of phys_to_virt() and have ISA range mappings continue through the
> > > > remaining ioremap() path.
> > > >
> > > > Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> > > > ---
> > > >    arch/x86/mm/ioremap.c |    7 +------
> > > >    1 file changed, 1 insertion(+), 6 deletions(-)
> > > >
> > > > diff --git a/arch/x86/mm/ioremap.c b/arch/x86/mm/ioremap.c
> > > > index 4c1b5fd..bfc3e2d 100644
> > > > --- a/arch/x86/mm/ioremap.c
> > > > +++ b/arch/x86/mm/ioremap.c
> > > > @@ -13,6 +13,7 @@
> > > >    #include <linux/slab.h>
> > > >    #include <linux/vmalloc.h>
> > > >    #include <linux/mmiotrace.h>
> > > > +#include <linux/mem_encrypt.h>
> > > >
> > > >    #include <asm/set_memory.h>
> > > >    #include <asm/e820/api.h>
> > > > @@ -106,12 +107,6 @@ static void __iomem
> > > > *__ioremap_caller(resource_size_t phys_addr,
> > > >           }
> > > >
> > > >           /*
> > > > -        * Don't remap the low PCI/ISA area, it's always mapped..
> > > > -        */
> > > > -       if (is_ISA_range(phys_addr, last_addr))
> > > > -               return (__force void __iomem *)phys_to_virt(phys_addr);
> > > > -
> > > > -       /*
> > > >            * Don't allow anybody to remap normal RAM that we're using..
> > > >            */
> > > >           pfn      = phys_addr >> PAGE_SHIFT;
> > > Removing this also affects 32-bit, which is more likely to access
> > > legacy devices in this range.  Put in a check for SME instead
> >
> > I originally had a check for SME here in a previous version of the
> > patch.  Thomas Gleixner recommended removing the check so that the code
> > path was always exercised regardless of the state of SME in order to
> > better detect issues:
> >
> > http://marc.info/?l=linux-kernel&m=149803067811436&w=2
> >
> > Thanks,
> > Tom
> Looking a bit closer, this shortcut doesn't set the caching
> attributes.  So it's probably best to get rid of it anyways.  Also
> note, there is a corresponding check in iounmap().
Good catch.  I'll update the patch to include the removal of the ISA
checks in the iounmap() path as well.

Thanks,
Tom

> --
> Brian Gerst
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