Messages in this thread | | | Date | Thu, 25 Jun 2020 11:37:20 -0400 | From | Qian Cai <> | Subject | Re: [PATCH 2/2] iommu/amd: Use 'unsigned long' for domain->pt_root |
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On Thu, Jun 25, 2020 at 04:52:27PM +0200, Joerg Roedel wrote: > From: Joerg Roedel <jroedel@suse.de> > > Using atomic64_t can be quite expensive, so use unsigned long instead. > This is safe because the write becomes visible atomically. > > Signed-off-by: Joerg Roedel <jroedel@suse.de> > --- > drivers/iommu/amd/amd_iommu_types.h | 2 +- > drivers/iommu/amd/iommu.c | 10 ++++++++-- > 2 files changed, 9 insertions(+), 3 deletions(-) > > diff --git a/drivers/iommu/amd/amd_iommu_types.h b/drivers/iommu/amd/amd_iommu_types.h > index 30a5d412255a..f6f102282dda 100644 > --- a/drivers/iommu/amd/amd_iommu_types.h > +++ b/drivers/iommu/amd/amd_iommu_types.h > @@ -468,7 +468,7 @@ struct protection_domain { > iommu core code */ > spinlock_t lock; /* mostly used to lock the page table*/ > u16 id; /* the domain id written to the device table */ > - atomic64_t pt_root; /* pgtable root and pgtable mode */ > + unsigned long pt_root; /* pgtable root and pgtable mode */ > int glx; /* Number of levels for GCR3 table */ > u64 *gcr3_tbl; /* Guest CR3 table */ > unsigned long flags; /* flags to find out type of domain */ > diff --git a/drivers/iommu/amd/iommu.c b/drivers/iommu/amd/iommu.c > index 5286ddcfc2f9..b0e1dc58244e 100644 > --- a/drivers/iommu/amd/iommu.c > +++ b/drivers/iommu/amd/iommu.c > @@ -156,7 +156,7 @@ static struct protection_domain *to_pdomain(struct iommu_domain *dom) > static void amd_iommu_domain_get_pgtable(struct protection_domain *domain, > struct domain_pgtable *pgtable) > { > - 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);
> + > + /* > + * The new value needs to be gobally visible in case pt_root gets > + * cleared, so that the page-table can be safely freed. > + */ > + smp_wmb(); > } > > static void amd_iommu_domain_clr_pt_root(struct protection_domain *domain) > -- > 2.27.0 >
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