Re: [PATCH] iommu/qcom: Invalidate TLB during context init

[Sam Day]

Hello Robin,

On Monday, 28 September 2026 at 10:22 PM, Robin Murphy <robin.murphy@xxxxxxx> wrote:

> On 28/09/2026 7:27 am, Sam Day via B4 Relay wrote:
> > From: Sam Day <me@xxxxxxxxxxx>
> >
> > qcom_iommu derives each context bank's ASID from DT data, so a kernel
> > booted via kexec reuses the same ASIDs as its predecessor.
> >
> > Programming a new TTBR0 doesn't discard entries the SMMU has already
> > cached under that ASID, so these residual and stale
> > translations/table-walks will go into effect as soon as the context is
> > enabled again.
> >
> > On MSM8916 devices (I confirmed it on both a Samsung Galaxy A5 and a
> > DragonBoard 410c) the result is MDP5 taking context faults on
> > framebuffer IOVAs and stuck in a continuous underrun storm during scan
> > out, if the previous kernel had itself initialized the display and
> > programmed the SMMU.
> >
> > arm-smmu invalidates the whole TLB in arm_smmu_device_reset() before
> > enabling the SMMU. qcom_iommu can't do that on these TZ-managed devices
> > (SMMU_SCR1.GASRAE=1), however.
>
> Can you not hit SMMU_CBn_TLBIALL in qcom_iommu_ctx_probe()? That would
> seem like the logical equivalent.

That sounds much more civilized, assuming it works the way we hope :) I will
respin the patch and test to see if SMMU_CBn_TLBIALL behaves correctly on
my apq8016-sbc + samsung-a5u-eur.

Kind regards,
-Sam

>
> Thanks,
> Robin.
>
> > Instead, qcom_iommu now invalidates each context bank by ASID whilst it
> > is still disabled, before programming it for the new domain. Secured
> > contexts are protected by TZ so they're skipped.
> >
> > There's been previous discussion on the list (see link) about how to
> > best deal with this kind of situation. This patch opts for a fix in the
> > incoming kernel, rather than the outgoing one. This ensures newer
> > kernels will always behave correctly, and also covers the kdump use
> > case.
> >
> > Fixes: 0ae349a0f33f ("iommu/qcom: Add qcom_iommu")
> > Link: https://lore.kernel.org/all/20240319154756.GB2901@willie-the-truck/
> > Assisted-by: LLM
> > Signed-off-by: Sam Day <me@xxxxxxxxxxx>
> > ---
> > Tested on my DragonBoard 410c. Starting from one unpatched kernel
> > scanning out at 640x480 and kexecing into the same kernel at 1280x720
> > results in context faults starting at the first IOVA past the previous
> > kernel's mapped extent, with continuous MDP5 underruns thereafter.
> > During this time I observed an all-blue HDMI signal. With the patch
> > applied, the same kexec hop is free of faults, and the HDMI signal is
> > clean throughout.
> >
> > Further, it was proven that kexecing into an unpatched kernel and
> > causing the fault storm can then be resolved by subsequently kexecing
> > into a patched kernel.
> > ---
> >   drivers/iommu/arm/arm-smmu/qcom_iommu.c | 32 +++++++++++++++++++++-----------
> >   1 file changed, 21 insertions(+), 11 deletions(-)
> >
> > diff --git a/drivers/iommu/arm/arm-smmu/qcom_iommu.c b/drivers/iommu/arm/arm-smmu/qcom_iommu.c
> > index 21d18ce67b982..a37955cd90d5e 100644
> > --- a/drivers/iommu/arm/arm-smmu/qcom_iommu.c
> > +++ b/drivers/iommu/arm/arm-smmu/qcom_iommu.c
> > @@ -111,23 +111,26 @@ iommu_readq(struct qcom_iommu_ctx *ctx, unsigned reg)
> >   	return readq_relaxed(ctx->base + reg);
> >   }
> >
> > +static void qcom_iommu_ctx_tlb_sync(struct qcom_iommu_ctx *ctx)
> > +{
> > +	unsigned int val, ret;
> > +
> > +	iommu_writel(ctx, ARM_SMMU_CB_TLBSYNC, 0);
> > +
> > +	ret = readl_poll_timeout(ctx->base + ARM_SMMU_CB_TLBSTATUS, val,
> > +				 (val & 0x1) == 0, 0, 5000000);
> > +	if (ret)
> > +		dev_err(ctx->dev, "timeout waiting for TLB SYNC\n");
> > +}
> > +
> >   static void qcom_iommu_tlb_sync(void *cookie)
> >   {
> >   	struct qcom_iommu_domain *qcom_domain = cookie;
> >   	struct iommu_fwspec *fwspec = qcom_domain->fwspec;
> >   	unsigned i;
> >
> > -	for (i = 0; i < fwspec->num_ids; i++) {
> > -		struct qcom_iommu_ctx *ctx = to_ctx(qcom_domain, fwspec->ids[i]);
> > -		unsigned int val, ret;
> > -
> > -		iommu_writel(ctx, ARM_SMMU_CB_TLBSYNC, 0);
> > -
> > -		ret = readl_poll_timeout(ctx->base + ARM_SMMU_CB_TLBSTATUS, val,
> > -					 (val & 0x1) == 0, 0, 5000000);
> > -		if (ret)
> > -			dev_err(ctx->dev, "timeout waiting for TLB SYNC\n");
> > -	}
> > +	for (i = 0; i < fwspec->num_ids; i++)
> > +		qcom_iommu_ctx_tlb_sync(to_ctx(qcom_domain, fwspec->ids[i]));
> >   }
> >
> >   static void qcom_iommu_tlb_inv_context(void *cookie)
> > @@ -270,6 +273,13 @@ static int qcom_iommu_init_domain(struct iommu_domain *domain,
> >   		/* Disable context bank before programming */
> >   		iommu_writel(ctx, ARM_SMMU_CB_SCTLR, 0);
> >
> > +		/* The TLB may still hold cached and stale entries for this
> > +		 * ASID, if a previous kernel programmed the SMMU before
> > +		 * a kexec into this kernel.
> > +		 */
> > +		iommu_writel(ctx, ARM_SMMU_CB_S1_TLBIASID, ctx->asid);
> > +		qcom_iommu_ctx_tlb_sync(ctx);
> > +
> >   		/* Clear context bank fault address fault status registers */
> >   		iommu_writel(ctx, ARM_SMMU_CB_FAR, 0);
> >   		iommu_writel(ctx, ARM_SMMU_CB_FSR, ARM_SMMU_CB_FSR_FAULT);
> >
> > ---
> > base-commit: 3339792beb5fb1c9c423c544ba2fbc235e7d7f75
> > change-id: 20260926-qcom-iommu-clean-contexts-3ccda804c08d
> >
> > Best regards,
>
>