Messages in this thread | | | Date | Fri, 19 Aug 2016 10:52:00 -0400 | From | Konrad Rzeszutek Wilk <> | Subject | Re: XSA 154 and ISA region (640K -> 1MB) WB cache instead of UC |
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On Thu, Aug 18, 2016 at 04:06:33AM -0600, Jan Beulich wrote: > >>> On 17.08.16 at 22:32, <konrad.wilk@oracle.com> wrote: > > One of the interesting things about XSA 154 fix ("x86: enforce consistent > > cachability of MMIO mappings") is that when certain applications (mcelog) > > are trying to map /dev/mmap and lurk in ISA regions - we get: > > DYM /dev/mem ? Most accesses to which are bogus in PV guests > (often including Dom0) anyway.
Yes. > > > [ 49.399053] WARNING: CPU: 0 PID: 2471 at arch/x86/mm/pat.c:913 untrack_pfn+0x93/0xc0() > > What Linux version is this? untrack_pfn() doesn't span line 913 in
4.1 > 4.8-rc2. And follow_phys() appears to only check whether the write > flag is set as expected; I can't see any cachability checks. Plus it > gets called only when both incoming address and size are zero.
The error that happens is much sooner - that is when the VMA is setup with the incorrect page attributes. Specifically: reserve_memtype which
548 /* Low ISA region is always mapped WB in page table. No need to track */ 549 if (x86_platform.is_untracked_pat_range(start, end)) { 550 if (new_type) 551 *new_type = _PAGE_CACHE_MODE_WB; 552 return 0; 553 }
(And this for a change is v4.8-rc2) > > > Anyhow what I am wondering: > > > > a) Should we add a edge case in the hypervisor to allow multiple mappings > > for this region? I am thinking no.. but it sounds like mapping ISA region > > as WB is safe even in baremetal? > > We should never allow multiple mappings with different cachability. > And I don't understand what makes you think the ISA region is safe > to map WB? There might be ROMs, MMIO regions, or simply nothing > there, neither of which is safe to map WB. ROMs certainly could be > WP, but that would require a way to reliably size not only ISA > extension ROMs, but also main and video BIOS. > > > b) Or would it be better to let Linux do its thing and treat 640KB->1MB > > as uncached instead of writeback? > > According to what you wrote earlier the two parts of the sentence > read contradictory to me. > > > Looking at the kernel it assumes that WB is ok for 640KB->1MB. > > The comment says: > > " /* Low ISA region is always mapped WB in page table. No need to track *" > > As per above it's not clear to me what this comment is backed by.
I was hoping you would know :-)
Ah, commit 2e5d9c857d4e6c9e7b7d8c8c86a68a7842d213d6 Author: venkatesh.pallipadi@intel.com <venkatesh.pallipadi@intel.com> Date: Tue Mar 18 17:00:14 2008 -0700
x86: PAT infrastructure patch Sets up pat_init() infrastructure.
which sets the MTRR for that region. > > Jan >
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