Re: [PATCH] soc: qcom: cmd-db: map shared memory as WT, not WB

[Nikita Travkin]

On Wed, Mar 27, 2024 at 08:09:34PM +0000, 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.
> 

Hi!

I observe a similar issue while trying to boot Linux in EL2 after taking
over qcom's hyp on a sc7180 WoA device:

[    0.337736] CPU: All CPU(s) started at EL2
(...)
[    0.475135] Serial: AMBA PL011 UART driver
[    0.479649] Internal error: synchronous external abort: 0000000096000410 [#1] PREEMPT SMP
[    0.488053] Modules linked in:
[    0.491213] CPU: 6 PID: 1 Comm: swapper/0 Not tainted 6.7.0 #41
[    0.497310] Hardware name: Acer Aspire 1 (DT)
[    0.501800] pstate: 00400009 (nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[    0.508964] pc : cmd_db_dev_probe+0x38/0xc4
[    0.513290] lr : cmd_db_dev_probe+0x2c/0xc4
[    0.517606] sp : ffff8000817ebab0
[    0.521019] x29: ffff8000817ebab0 x28: 0000000000000000 x27: ffff800081346050
                     <uart cuts out>

Unfortunately this patch doesn't help in this case (I beileve I even
tried same/similar change a while back when trying to debug this)

Currently I can work around this by just reocationg the cmd-db while
still under the qcom's hyp [1] but it would be nice to find a generic
solution that doesn't need pre-boot hacks...

AFAIK this is not observed on at least sc8280xp WoA devices and I'd
assume cros is not affected because they don't use qcom's TZ and instead
use TF-A (which is overall more friendly, though still uses qcom's
proprietary qtiseclib under the hood)

Nikita

[1] https://github.com/TravMurav/slbounce/blob/main/src/dtbhack_main.c#L17

> Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>
> ---
>  drivers/soc/qcom/cmd-db.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/soc/qcom/cmd-db.c b/drivers/soc/qcom/cmd-db.c
> index a5fd68411bed5..dd5ababdb476c 100644
> --- a/drivers/soc/qcom/cmd-db.c
> +++ b/drivers/soc/qcom/cmd-db.c
> @@ -324,7 +324,7 @@ static int cmd_db_dev_probe(struct platform_device *pdev)
>  		return -EINVAL;
>  	}
>  
> -	cmd_db_header = memremap(rmem->base, rmem->size, MEMREMAP_WB);
> +	cmd_db_header = memremap(rmem->base, rmem->size, MEMREMAP_WT);
>  	if (!cmd_db_header) {
>  		ret = -ENOMEM;
>  		cmd_db_header = NULL;
> -- 
> 2.43.0