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Date:   Sat, 2 Oct 2021 21:14:48 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Ajay Garg <ajaygargnsit@...il.com>, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, kvm@...r.kernel.org
Cc:     baolu.lu@...ux.intel.com, "Tian, Kevin" <kevin.tian@...el.com>
Subject: Re: [PATCH] iommu: intel: remove flooding of non-error logs, when
 new-DMA-PTE is the same as old-DMA-PTE.

On 2021/10/2 20:40, Ajay Garg wrote:
> Taking a SD-MMC controller (over PCI) as an example, following is an
> example sequencing, where the log-flooding happened :
> 
> 0.
> We have a host and a guest, both running latest x86_64 kernels.
> 
> 1.
> Host-machine is booted up (with intel_iommu=on), and the DMA-PTEs
> are setup for the controller (on the host), for the first time.
> 
> 2.
> The SD-controller device is added to a (L1) guest on a KVM-VM
> (via virt-manager).

Isn't the domain should be switched from a default domain to an
unmanaged domain when the device is assigned to the guest?

Even you want to r-setup the same mappings, you need to un-map all
existing mappings, right?

Best regards,
baolu

> 
> 3.
> The KVM-VM is booted up.
> 
> 4.
> Above triggers a re-setup of DMA-PTEs on the host, for a
> second time.
> 
> It is observed that the new PTEs formed (on the host) are same
> as the original PTEs, and thus following logs, accompanied by
> stacktraces, overwhelm the logs :
> 
> ......
>   DMAR: ERROR: DMA PTE for vPFN 0x428ec already set (to 3f6ec003 not 3f6ec003)
>   DMAR: ERROR: DMA PTE for vPFN 0x428ed already set (to 3f6ed003 not 3f6ed003)
>   DMAR: ERROR: DMA PTE for vPFN 0x428ee already set (to 3f6ee003 not 3f6ee003)
>   DMAR: ERROR: DMA PTE for vPFN 0x428ef already set (to 3f6ef003 not 3f6ef003)
>   DMAR: ERROR: DMA PTE for vPFN 0x428f0 already set (to 3f6f0003 not 3f6f0003)
> ......
> 
> As the PTEs are same, so there is no cause of concern, and we can easily
> avoid the logs-flood for this non-error case.
> 
> Signed-off-by: Ajay Garg <ajaygargnsit@...il.com>
> ---
>   drivers/iommu/intel/iommu.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index d75f59ae28e6..8bea8b4e3ff9 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -2370,7 +2370,7 @@ __domain_mapping(struct dmar_domain *domain, unsigned long iov_pfn,
>   		 * touches the iova range
>   		 */
>   		tmp = cmpxchg64_local(&pte->val, 0ULL, pteval);
> -		if (tmp) {
> +		if (tmp && (tmp != pteval)) {
>   			static int dumps = 5;
>   			pr_crit("ERROR: DMA PTE for vPFN 0x%lx already set (to %llx not %llx)\n",
>   				iov_pfn, tmp, (unsigned long long)pteval);
> 

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