On 26.08.25 10:38, Christian König wrote:
On 25.08.25 21:10, David Hildenbrand wrote:
On 21.08.25 10:10, Christian König wrote:
On 20.08.25 17:23, David Hildenbrand wrote:
CCing Lorenzo
On 20.08.25 16:33, Christian König wrote:
Hi everyone,
sorry for CCing so many people, but that rabbit hole turned out to be
deeper than originally thought.
TTM always had problems with UC/WC mappings on 32bit systems and drivers
often had to revert to hacks like using GFP_DMA32 to get things working
while having no rational explanation why that helped (see the TTM AGP,
radeon and nouveau driver code for that).
It turned out that the PAT implementation we use on x86 not only enforces
the same caching attributes for pages in the linear kernel mapping, but
also for highmem pages through a separate R/B tree.
That was unexpected and TTM never updated that R/B tree for highmem pages,
so the function pgprot_set_cachemode() just overwrote the caching
attributes drivers passed in to vmf_insert_pfn_prot() and that essentially
caused all kind of random trouble.
An R/B tree is potentially not a good data structure to hold thousands if
not millions of different attributes for each page, so updating that is
probably not the way to solve this issue.
Thomas pointed out that the i915 driver is using apply_page_range()
instead of vmf_insert_pfn_prot() to circumvent the PAT implementation and
just fill in the page tables with what the driver things is the right
caching attribute.
I assume you mean apply_to_page_range() -- same issue in patch subjects.
Oh yes, of course. Sorry.
Oh this sounds horrible. Why oh why do we have these hacks in core-mm and have
drivers abuse them :(
Yeah I was also a bit hesitated to use that, but the performance advantage is
so high that we probably can't avoid the general approach.
Honestly, apply_to_pte_range() is just the entry in doing all kinds of weird crap to page
tables because "you know better".
Exactly that's the problem I'm pointing out, drivers *do* know it better. The
core memory management has applied incorrect values which caused all kind of
the trouble.
The problem is not a bug in PAT nor TTM/drivers but rather how they interact
with each other.
What I don't understand is why do we have the PAT in the first place? No other
architecture does it this way.
Probably because no other architecture has these weird glitches I assume ...
skimming over memtype_reserve() and friends there are quite some corner cases
the code is handling (BIOS, ACPI, low ISA, system RAM, ...)
I did a lot of work on the higher PAT level functions, but I am no expert on
the lower level management functions, and in particular all the special cases
with different memory types.
IIRC, the goal of the PAT subsystem is to make sure that no two page tables map
the same PFN with different caching attributes.
Yeah, that actually makes sense. Thomas from Intel recently explained the
technical background to me:
Some x86 CPUs write back cache lines even if they aren't dirty and what can
happen is that because of the linear mapping the CPU speculatively loads a
cache line which is elsewhere mapped uncached.
So the end result is that the writeback of not dirty cache lines potentially
corrupts the data in the otherwise uncached system memory.
But that a) only applies to memory in the linear mapping and b) only to a
handful of x86 CPU types (e.g. recently Intels Luna Lake, AMD Athlons produced
before 2004, maybe others).
It treats ordinary system RAM (IORESOURCE_SYSTEM_RAM) usually in a special way:
no special caching mode.
For everything else, it expects that someone first reserves a memory range for
a specific caching mode.
For example, remap_pfn_range()...->pfnmap_track()->memtype_reserve() will make
sure that there are no conflicts, to the call memtype_kernel_map_sync() to make sure
the identity mapping is updated to the new type.
In case someone ends up calling pfnmap_setup_cachemode(), the expectation is
that there was a previous call to memtype_reserve_io() or similar, such that
pfnmap_setup_cachemode() will find that caching mode.
So my assumption would be that that is missing for the drivers here?
Well yes and no.
See the PAT is optimized for applying specific caching attributes to ranges
[A..B] (e.g. it uses an R/B tree). But what drivers do here is that they have
single pages (usually for get_free_page or similar) and want to apply a certain
caching attribute to it.
One clarification after staring at PAT code once again: for pages (RAM),
the caching attribute is stored in the page flags, not in the R/B tree.
If nothing was set, it defaults to _PAGE_CACHE_MODE_WB AFAIKs.
--
Cheers
David / dhildenb
