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

[Tom Lendacky <thomas.lendacky@xxxxxxx>]

On 7/11/2017 10:38 AM, Brian Gerst wrote:
> On Tue, Jul 11, 2017 at 11:02 AM, Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:
> > 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.
> I now think it should be kept but also emit a warning, at least for
> the short term.  There is bad code out there (vga16fb for example)
> that calls iounmap() blindly without calling ioremap() first.  We
> don't want to actually follow through with the unmap on the linear
> mapping.
Yup, was just about to reply to the other email on this. That makes
sense, keep the check but add a warning to it so that it will catch
any misuses of iounmap() and those can then be addressed.

Thanks,
Tom

> --
> Brian Gerst
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