Re: [PATCH rc v7 0/7] iommu/arm-smmu-v3: Fix device crash on kdump kernel

[Mostafa Saleh <smostafa@google.com>]

On Tue, Jun 30, 2026 at 02:51:40PM +0000, Pranjal Shrivastava wrote:
> On Tue, Jun 30, 2026 at 01:17:30PM +0000, Mostafa Saleh wrote:
> > On Mon, Jun 29, 2026 at 11:15:33PM -0700, Nicolin Chen wrote:
> > > When transitioning to a kdump kernel, the primary kernel might have crashed
> > > while endpoint devices were actively bus-mastering DMA. Currently, the SMMU
> > > driver aggressively resets the hardware during probe by clearing CR0_SMMUEN
> > > and setting the Global Bypass Attribute (GBPA) to ABORT.
> > > 
> > > In a kdump scenario, this aggressive reset is highly destructive:
> > > a) If GBPA is set to ABORT, in-flight DMA will be aborted, generating fatal
> > >    PCIe AER or SErrors that may panic the kdump kernel
> > 
> > Can you please clarify more on those errors, what conditions will
> > trigger that?
> > For example, patch 4 disables the EVTQ to avoid events as there might
> > be a lot, why are they not fatal also?
> > 
> > > b) If GBPA is set to BYPASS, in-flight DMA targeting some IOVAs will bypass
> > >    the SMMU and corrupt the physical memory at those 1:1 mapped IOVAs.
> > > 
> > > To safely absorb in-flight DMA, the kdump kernel must leave SMMUEN=1 intact
> > > and avoid modifying STRTAB_BASE. This allows HW to continue translating in-
> > > flight DMA using the crashed kernel's page tables until the endpoint device
> > > drivers probe and quiesce their respective hardware.
> > > 
> > > However, the ARM SMMUv3 architecture specification states that updating the
> > > SMMU_STRTAB_BASE register while SMMUEN == 1 is UNPREDICTABLE or ignored.
> > > 
> > > This leaves a kdump kernel no choice but to adopt the stream table from the
> > > crashed kernel.
> > 
> > In many cases the patches assume that the CDs/STE might be corrupted,
> > but still attempt to retrieve them with some validation
> > (log2size/split...)
> > However, the base address might be broken, TLBs state is unknown...
> > 
> > IMO, although that might improve the status quo, there are still
> > heuristics, in addition to noticeable complexity to transition the
> > stream tables. I wonder if FW can deal with AER in that case before
> > booting the kdump kernel.
> 
> I guess we're reading the base address from the HW register itself so
> that should be fine? CDs are in-memory so that's why they could be
> corrupted?

For example patch#1 verifies log2size and split and both are read
from HW registers. Same for the base address or other addresses as
the page tables, they  might be corrupted due to a buggy driver.
My point is that, it is really hard to assume that the previous state
of registers/STE/page-tables were valid or even consistent, when the
kernel crashed and did not transition the state gracefully.

> 
> About the TLB state, I'm not sure what might pollute it, since this is a
> kexec, I don't expect any non-kernel entity to gain program control
> before the kdump kernel.. Hence, IMO, we can't configure FW to deal with
> AER here..

Similarly for TLBs, the kernel might have panicked in the middle of an
unmap or free domain. (not to mention what that means for RPM where
a device reset with unknown TLBs)

Why can't the FW deal with it? As I mentioned above in the previous
reply I am not sure I understand what situation leads into this, when
does a device trigger SError to the system vs when not which is observed
as an event in that case.

Thanks,
Mostafa

> 
> Thanks,
> Praan