linux-kernel - Re: [PATCH] iommu/vt-d: Fix PASID directory pointer coherency

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Date:   Tue, 7 Feb 2023 10:08:35 -0800
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, iommu@...ts.linux.dev,
        Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Raj Ashok <ashok.raj@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>, Yi Liu <yi.l.liu@...el.com>,
        stable@...r.kernel.org, Sukumar Ghorai <sukumar.ghorai@...el.com>,
        jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH] iommu/vt-d: Fix PASID directory pointer coherency

Hi Baolu,

On Tue, 7 Feb 2023 15:10:48 +0800, Baolu Lu <baolu.lu@...ux.intel.com>
wrote:

> On 2023/2/7 1:25, Jacob Pan wrote:
> >>> ---
> >>>    drivers/iommu/intel/iommu.c | 6 ++++++
> >>>    1 file changed, 6 insertions(+)
> >>>
> >>> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> >>> index 59df7e42fd53..b4878c7ac008 100644
> >>> --- a/drivers/iommu/intel/iommu.c
> >>> +++ b/drivers/iommu/intel/iommu.c
> >>> @@ -1976,6 +1976,12 @@ static int domain_context_mapping_one(struct
> >>> dmar_domain *domain, pds = context_get_sm_pds(table);
> >>>    		context->lo = (u64)virt_to_phys(table->table) |
> >>>    				context_pdts(pds);
> >>> +		/*
> >>> +		 * Scalable-mode PASID directory pointer is not
> >>> snooped if the
> >>> +		 * coherent bit is not set.
> >>> +		 */
> >>> +		if (!ecap_coherent(iommu->ecap))
> >>> +			clflush_cache_range(table->table, sizeof(void
> >>> *));  
> >> This isn't comprehensive. The clflush should be called whenever the
> >> pasid directory table is allocated or updated.
> >>  
> > allocate a pasid table does not mean it gets used by iommu hw, not
> > until it is programmed into context entry.  
> 
> Hi Jacob,
> 
> This page is used by the device, and the device's access to this memory
> is not coherent. So after the page is allocated, any changes made by the
> CPU to this page must be written back to the real memory.
> 
> This patch only flushes the first 8 bytes of the table. That's not
> enough.
> 
make sense, thanks for the explanation.
> Be aware that page allocation also requires a clflush, because at least
> __GFP_ ZERO implies modification to page.
> 
Yes, zeroing dir page does change it but I think if we intercept every
update to the directory entry, it should be sufficient?

will send an updated version.


Thanks,

Jacob