On Wed, Apr 10, 2024 at 10:12:37PM +0000, Volodymyr Babchuk wrote: > Stephan Gerhold <stephan@xxxxxxxxxxx> writes: > > On Wed, Mar 27, 2024 at 11:29:09PM +0000, Caleb Connolly wrote: > >> On 27/03/2024 21:06, Konrad Dybcio wrote: > >> > On 27.03.2024 10:04 PM, Volodymyr Babchuk wrote: > >> >> Konrad Dybcio <konrad.dybcio@xxxxxxxxxx> writes: > >> >>> On 27.03.2024 9:09 PM, Volodymyr Babchuk wrote: > >> >>>> It appears that hardware does not like cacheable accesses to this > >> >>>> region. Trying to access this shared memory region as Normal Memory > >> >>>> leads to secure interrupt which causes an endless loop somewhere in > >> >>>> Trust Zone. > >> >>>> > >> >>>> The only reason it is working right now is because Qualcomm Hypervisor > >> >>>> maps the same region as Non-Cacheable memory in Stage 2 translation > >> >>>> tables. The issue manifests if we want to use another hypervisor (like > >> >>>> Xen or KVM), which does not know anything about those specific > >> >>>> mappings. This patch fixes the issue by mapping the shared memory as > >> >>>> Write-Through. This removes dependency on correct mappings in Stage 2 > >> >>>> tables. > >> >>>> > >> >>>> I tested this on SA8155P with Xen. > >> >>>> > >> >>>> Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx> > >> >>>> --- > >> >>> > >> >>> Interesting.. > >> >>> > >> >>> +Doug, Rob have you ever seen this on Chrome? (FYI, Volodymyr, chromebooks > >> >>> ship with no qcom hypervisor) > >> >> > >> >> Well, maybe I was wrong when called this thing "hypervisor". All I know > >> >> that it sits in hyp.mbn partition and all what it does is setup EL2 > >> >> before switching to EL1 and running UEFI. > >> >> > >> >> In my experiments I replaced contents of hyp.mbn with U-Boot, which gave > >> >> me access to EL2 and I was able to boot Xen and then Linux as Dom0. > >> > > >> > Yeah we're talking about the same thing. I was just curious whether > >> > the Chrome folks have heard of it, or whether they have any changes/ > >> > workarounds for it. > >> > >> Does Linux ever write to this region? Given that the Chromebooks don't > >> seem to have issues with this (we have a bunch of them in pmOS and I'd > >> be very very surprised if this was an issue there which nobody had tried > >> upstreaming before) I'd guess the significant difference here is between > >> booting Linux in EL2 (as Chromebooks do?) vs with Xen. > >> > > > > FWIW: This old patch series from Stephen Boyd is closely related: > > https://urldefense.com/v3/__https://lore.kernel.org/linux-arm-msm/20190910160903.65694-1-swboyd@xxxxxxxxxxxx/__;!!GF_29dbcQIUBPA!yGecMHGezwkDU9t7XATVTI80PNGjZdQV2xsYFTl6EhpMMsRf_7xryKx8mEVpmTwTcKMGaaWomtyvr05zFcmsf2Kk$ > > [lore[.]kernel[.]org] > > > >> The main use case I have is to map the command-db memory region on > >> Qualcomm devices with a read-only mapping. It's already a const marked > >> pointer and the API returns const pointers as well, so this series > >> makes sure that even stray writes can't modify the memory. > > > > Stephen, what was the end result of that patch series? Mapping the > > cmd-db read-only sounds cleaner than trying to be lucky with the right > > set of cache flags. > > > > I checked the series, but I am afraid that I have no capacity to finish > this. Will it be okay to move forward with my patch? I understand that > this is not the best solution, but it is simple and it works. If this is > fine, I'll send v2 with all comments addressed. > My current understanding is that the important property here is to have a non-cacheable mapping, which is the case for both MEMREMAP_WT and MEMREMAP_WC, but not MEMREMAP_WB. Unfortunately, the MEMREMAP_RO option Stephen introduced is also a cacheable mapping, which still seems to trigger the issue in some cases. I'm not sure why a cache writeback still happens when the mapping is read-only and nobody writes anything. You can also test it if you want. For a quick test, - cmd_db_header = memremap(rmem->base, rmem->size, MEMREMAP_WB); + cmd_db_header = ioremap_prot(rmem->base, rmem->size, _PAGE_KERNEL_RO); should be (largely) equivalent to MEMREMAP_RO with Stephen's patch series. I asked Nikita to test this on SC7180 and it still seems to cause the crash. It seems to work only with a read-only non-cacheable mapping, e.g. with + cmd_db_header = ioremap_prot(rmem->base, rmem->size, ((PROT_NORMAL_NC & ~PTE_WRITE) | PTE_RDONLY)); The lines I just suggested for testing are highly architecture-specific though so not usable for a proper patch. If MEMREMAP_RO does not solve the real problem here then the work to make an usable read-only mapping would go beyond just finishing Stephen's patch series, since one would need to introduce some kind of MEMREMAP_RO_NC flag that creates a read-only non-cacheable mapping. It is definitely easier to just change the driver to use the existing MEMREMAP_WC. Given the crash you found, the hardware/firmware seems to have a built-in write protection on most platforms anyway. :D Thanks, Stephan
