Re: git: 060699e91369 - stable/13 - Merge llvm-project release/15.x llvmorg-15.0.7-0-g8dfdcc7b7bf6

On 1 May 2023, at 18:14, John Baldwin <jhb@freebsd.org> wrote:
> 
> On 4/30/23 8:31 PM, Jason A. Harmening wrote:
>> On Sun, Apr 30, 2023 at 07:34:45PM -0500, Jason A. Harmening wrote:
>>> On Sun, Apr 30, 2023 at 06:47:13PM -0500, Jason A. Harmening wrote:
>>>> On Sun, Apr 30, 2023 at 08:09:16AM +0300, Konstantin Belousov wrote:
>>>>> On Sat, Apr 29, 2023 at 02:27:50PM -0500, Jason A. Harmening wrote:
>>>>>> On Sat, Apr 29, 2023 at 08:49:28PM +0200, Dimitry Andric wrote:
>>>>>>> On 29 Apr 2023, at 20:33, Jason A. Harmening <jah@FreeBSD.org> wrote:
>>>>>>>> 
>>>>>>>> On Sun, Apr 09, 2023 at 09:35:22PM +0000, Dimitry Andric wrote:
>>>>>>>>> The branch stable/13 has been updated by dim:
>>>>>>>>> 
>>>>>>>>> URL: https://cgit.FreeBSD.org/src/commit/?id=060699e9136975d51d3f726b9785bdbac9a62ba6
>>>>>>>>> 
>>>>>>>>> commit 060699e9136975d51d3f726b9785bdbac9a62ba6
>>>>>>>>> Author:     Dimitry Andric <dim@FreeBSD.org>
>>>>>>>>> AuthorDate: 2023-01-14 16:33:24 +0000
>>>>>>>>> Commit:     Dimitry Andric <dim@FreeBSD.org>
>>>>>>>>> CommitDate: 2023-04-09 14:54:52 +0000
>>>>>>>>> 
>>>>>>>>>    Merge llvm-project release/15.x llvmorg-15.0.7-0-g8dfdcc7b7bf6
>>>>>>>>> 
>>>>>>>>>    This updates llvm, clang, compiler-rt, libc++, libunwind, lld, lldb and
>>>>>>>>>    openmp to llvmorg-15.0.7-0-g8dfdcc7b7bf6.
>>>>>>>>> 
>>>>>>>>>    PR:             265425
>>>>>>>>>    MFC after:      2 weeks
>>>>>>>> 
>>>>>>>> This MFC of llvm15 appears to have completely broken the Intel IOMMU
>>>>>>>> driver on my stable/13 machine.  After this series of commits, any
>>>>>>>> downstream DMA seems to produce an IOMMU translation fault, which
>>>>>>>> renders the machine completely unusable: no nvme boot disk, no usb
>>>>>>>> keyboard, etc.
>>>>>>>> 
>>>>>>>> The faults I see look something like this:
>>>>>>>> 
>>>>>>>> DMAR4: ahci0: pci0:17:5 sid 8d fault acc 0 adt 0x0 reason 0x3 addr 26000
>>>>>>>> 
>>>>>>>> It's a bit surprising to see a toolchain upgrade produce breakage like
>>>>>>>> this, but that's what git bisect clearly tells me.  I wonder if some of
>>>>>>>> the IOMMU control structures might be defined as C bitfields and the new
>>>>>>>> compiler is emitting them differently?  Also, was any breakage like this
>>>>>>>> observed when -current was upgraded to llvm15 several months ago?
>>>>>>> 
>>>>>>> I haven't heard anything about such breakage, no.
>>>>>>> 
>>>>>>> 
>>>>>>>> More generally, this is the second time in as many months I've had to
>>>>>>>> deal with IOMMU breakage on -stable.  I can't imagine I'm the only
>>>>>>>> person who sees value in running with DMA remapping enabled; do we need
>>>>>>>> a dedicated DMAR-enabled machine in the cluster to smoke-test changes
>>>>>>>> like this?  More generally, should we avoid MFCing high-risk changes
>>>>>>>> like this?
>>>>>>> 
>>>>>>> Since there were very few bug reports, it was not deemed high risk.
>>>>>>> 
>>>>>>> In any case, it would be good to get the bottom of what is causing the
>>>>>>> problem, so is there any way you can isolate which code seems to be
>>>>>>> going "bad"?
>>>>>>> 
>>>>>>> For example, if this problem affects code in sys/dev/iommu, is there
>>>>>>> some way you can compile that part with -O1, or with an older version
>>>>>>> of clang (from ports), to see if the problem goes away?
>>>>>> 
>>>>>> I did try removing all custom make.conf settings (previously I just had
>>>>>> CPUTYPE?=icelake-server), but that didn't change the behavior.
>>>>>> 
>>>>>> Before I try further build tweaks, I'd like to ask if the IOMMU fault
>>>>>> report can provide guidance here?  AFAICT all the faults I'm getting
>>>>>> show "reason 0x3".  If I'm reading the VT-d spec correctly, FR=0x3
>>>>>> indicates an invalid context entry, in other words there's something the
>>>>>> hardware doesn't like in the way the address width or pagetable base is
>>>>>> configured for the PCIe requestor.
>>>>> 
>>>>> I would start looking at the other direction: might be, there are still some
>>>>> left shifts for int32 values with the shift count > 30, or uint32 with the
>>>>> count > 31.
>>>>> 
>>>>> Also might be useful to dump each context entry on creation, it is kept
>>>>> constant after.
>>>> 
>>>> I did look over the constants in intel_reg.h, and didn't see anything
>>>> that looked as though it would be susceptible to sign-extension or
>>>> truncation bugs.  In the failing case it's much easier for me to catch
>>>> the fault messages than any initialization message, so I instrumented
>>>> the fault handler to get the context entry from the dmar_ctx object
>>>> using the same logic as dmar_map_ctx_entry(), and then dump out the ctx1
>>>> and ctx2 fields.  What I see are messages like:
>>>> 
>>>> ... ctx1 0x10013b001 ctx2 0x103
>>>> 
>>>> At first glance these "look right": the P bit is set in ctx1, and the
>>>> rest of the field looks like a valid physical address.  ctx2 also
>>>> doesn't have any of the reserved bits set, but in all cases it does have
>>>> AW=3, which would indicate 57-bit AGAW.  But when I boot the last
>>>> working kernel, from the revision prior to the llvm15 MFC, I see this in
>>>> dmesg:
>>>> 
>>>> ahci0: dmar4 pci0:0:17:5 rid 8d domain 1 mgaw 48 agaw 48 re-mapped
>>>> 
>>>> ...all reported devices show 48-bit MGAW/AGAW, so I would expect ctx2 to
>>>> have AW=2.  I suspect this may be the source of the fault, but I'm not
>>>> sure how it's getting configured that way, whether it's an issue with
>>>> reading the capability register or something else.
>>>> 
>>> 
>>> I can confirm that hacking domain_set_agaw() to always use the settings
>>> from sagaw_bits[2] eliminates the faults and at least allows the machine
>>> to boot to single-user mode.
>> I see what's happening now.  When I added the hack to always set
>> sagaw_bits[2], I noted that the passed-in MGAW was still 57, while
>> unit->hw_cap had the correct value of 0x4 (=> 4-level paging, 48-bit AW)
>> in bits 12:8.  The problem is that sagaw_bits has agaw=64 in its last
>> entry.  This results in dmar_maxaddr2mgaw() attempting a comparison
>> against 1ULL << 64 in the final iteration of its first loop.  I suspect
>> the new compiler probably determines that last iteration is meaningless
>> and simply omits it from the (probably unrolled) loop.  Since the "loop"
>> terminates with i < nitems(sagaw_bits), the subsequent "allow_less ..."
>> case doesn't execute and we end up erroneously selecting a 57-bit
>> address width.  Just commenting out that last entry in sagaw_bits fixes
>> the problem.
>> So, two questions:
>> 1) Does any VT-d hardware actually support 6-level paging?  The ca. 2021
>> VT-d spec I'm looking at indicates 5-level is the greatest depth
>> supported, with everything above that being reserved.
>> 2) I'd expect clang to try very hard to error out in a situation like
>> this, but I see that sys/conf/kern.mk sets -Wno-shift-count-overflow
>> among other things, and more of them were added for clang 15.  This
>> seems like a really bad idea, regardless of how much of a PITA it may be
>> to fix these warnings.
> 
> FWIW, I've been working on trying to re-enable some of the warnings that
> were disabled for clang 15 in main.  I'll move that one higher up on my
> todo list.

In this particular case, it doesn't warn about it though. I think KASAN
might be a better 'catcher' for this kind of error, or a KUBSAN, if we
had one...

-Dimitry