linux-kernel - Re: [PATCH 2/2] iommu/amd: Use 'unsigned long' for domain->pt

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Date:   Fri, 26 Jun 2020 10:05:40 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Qian Cai <cai@....pw>
Cc:     iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 2/2] iommu/amd: Use 'unsigned long' for domain->pt_root

Hi Qian,

On Thu, Jun 25, 2020 at 11:37:20AM -0400, Qian Cai wrote:
> On Thu, Jun 25, 2020 at 04:52:27PM +0200, Joerg Roedel wrote:
> > -	u64 pt_root = atomic64_read(&domain->pt_root);
> > +	unsigned long pt_root = domain->pt_root;
> 
> The pt_root might be reload later in case of register pressure where the
> compiler decides to not store it as a stack variable, so it needs
> smp_rmb() here to match to the smp_wmb() in
> amd_iommu_domain_set_pt_root() to make the load visiable to all CPUs.
> 
> Then, smp_rmb/wmb() wouldn't be able to deal with data races, so it
> needs,
> 
> unsigned long pt_root = READ_ONCE(domain->pt_root);
> 
> >  
> >  	pgtable->root = (u64 *)(pt_root & PAGE_MASK);
> >  	pgtable->mode = pt_root & 7; /* lowest 3 bits encode pgtable mode */
> > @@ -164,7 +164,13 @@ static void amd_iommu_domain_get_pgtable(struct protection_domain *domain,
> >  
> >  static void amd_iommu_domain_set_pt_root(struct protection_domain *domain, u64 root)
> >  {
> > -	atomic64_set(&domain->pt_root, root);
> > +	domain->pt_root = root;
> 
> WRITE_ONCE(domain->pt_root, root);

Thanks for your review. I addressed your comments and will send an
updated version shortly.


Regards,

	Joerg