Re: [PATCH v2 4/4] iommu/hyperv: Add page-selective IOTLB flush support

[Yu Zhang <zhangyu1@linux.microsoft.com>]

On Thu, Jul 09, 2026 at 07:08:47PM +0000, Michael Kelley wrote:
> From: Yu Zhang <zhangyu1@linux.microsoft.com> Sent: Thursday, July 2, 2026 9:05 AM
> > 
> > Add page-selective IOTLB flush using HVCALL_FLUSH_DEVICE_DOMAIN_LIST.
> > This hypercall accepts a list of (page_number, page_mask_shift) entries,
> > enabling finer-grained IOTLB invalidation compared to the domain-wide
> > HVCALL_FLUSH_DEVICE_DOMAIN used by hv_iommu_flush_iotlb_all().
> > 
> > hv_iommu_calc_flush_range() computes the smallest power-of-two aligned
> > range that covers the target IOVA region, producing a single flush
> > descriptor. This may over-flush when the range is not naturally aligned,
> > matching the approach used by Intel VT-d PSI. If the page-selective
> > flush fails, the code falls back to a full domain flush.
> > 
> > Signed-off-by: Easwar Hariharan <easwar.hariharan@linux.microsoft.com>
> > Signed-off-by: Yu Zhang <zhangyu1@linux.microsoft.com>
> > ---
> >  drivers/iommu/hyperv/iommu.c | 68 +++++++++++++++++++++++++++++++++++-
> >  include/hyperv/hvgdk_mini.h  |  1 +
> >  include/hyperv/hvhdk_mini.h  | 17 +++++++++
> >  3 files changed, 85 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/iommu/hyperv/iommu.c b/drivers/iommu/hyperv/iommu.c
> > index 254136946404..e9b104a322fd 100644
> > --- a/drivers/iommu/hyperv/iommu.c
> > +++ b/drivers/iommu/hyperv/iommu.c
> > @@ -9,6 +9,7 @@
> >  #define pr_fmt(fmt) "Hyper-V pvIOMMU: " fmt
> >  #define dev_fmt(fmt) pr_fmt(fmt)
> > 
> > +#include <linux/hyperv.h>
> >  #include <linux/iommu.h>
> >  #include <linux/pci.h>
> >  #include <linux/dma-map-ops.h>
> > @@ -401,10 +402,74 @@ static void hv_iommu_flush_iotlb_all(struct iommu_domain *domain)
> >  	hv_flush_device_domain(to_hv_iommu_domain(domain));
> >  }
> > 
> > +/*
> > + * Calculate the minimal power-of-two aligned range that covers [start, end]
> > + * (end is inclusive). Returns a single (page_number, page_mask_shift)
> > + * descriptor that may over-flush when the range is not naturally aligned.
> > + */
> > +static void hv_iommu_calc_flush_range(unsigned long start, unsigned long end,
> > +				       union hv_iommu_flush_va *va)
> > +{
> > +	unsigned long start_pfn = HVPFN_DOWN(start);
> > +	unsigned long last_pfn = HVPFN_UP(end + 1) - 1;
> > +	unsigned long mask_shift, aligned_pfn;
> > +
> > +	if (start_pfn == last_pfn) {
> > +		mask_shift = 0;
> > +	} else {
> > +		/*
> > +		 * Find the highest bit position where start_pfn and last_pfn
> > +		 * differ.  A range aligned to one above that bit is the
> > +		 * smallest power-of-two region that covers both endpoints.
> > +		 */
> > +		mask_shift = __fls(start_pfn ^ last_pfn) + 1;
> > +	}
> > +
> > +	aligned_pfn = ALIGN_DOWN(start_pfn, 1UL << mask_shift);
> > +	va->page_number = aligned_pfn;
> > +	va->page_mask_shift = mask_shift;
> > +}
> > +
> > +static void hv_flush_device_domain_list(struct hv_iommu_domain *hv_domain,
> > +					struct iommu_iotlb_gather *iotlb_gather)
> > +{
> > +	u64 status;
> > +	unsigned long flags;
> > +	struct hv_input_flush_device_domain_list *input;
> > +
> > +	local_irq_save(flags);
> > +
> > +	input = *this_cpu_ptr(hyperv_pcpu_input_arg);
> > +	memset(input, 0, sizeof(*input));
> > +
> > +	input->device_domain = hv_domain->device_domain;
> > +	input->flags |= HV_FLUSH_DEVICE_DOMAIN_LIST_IOMMU_FORMAT;
> > +	hv_iommu_calc_flush_range(iotlb_gather->start, iotlb_gather->end,
> > +				  &input->iova_list[0]);
> > +
> > +	status = hv_do_rep_hypercall(HVCALL_FLUSH_DEVICE_DOMAIN_LIST,
> > +				     1, 0, input, NULL);
> > +
> > +	if (!hv_result_success(status)) {
> > +		/* Page-selective flush failed, fall back to full flush. */
> 
> With the selective flush now simplified to just a single entry, it really
> shouldn't fail, right? Doing a full flush as a fallback makes sense, but
> perhaps do a WARN_ON_ONCE() first so that there's an indication that
> the selective flush failed.
> 

It shouldn't fail under normal circumstances. Will add a WARN_ON_ONCE()
in case the fallback happens. Thanks!

> > +		struct hv_input_flush_device_domain *flush_all = (void *)input;
> > +
> > +		memset(flush_all, 0, sizeof(*flush_all));
> > +		flush_all->device_domain = hv_domain->device_domain;
> > +		status = hv_do_hypercall(HVCALL_FLUSH_DEVICE_DOMAIN,
> > +					flush_all, NULL);
> > +		WARN(!hv_result_success(status),
> > +		     "HVCALL_FLUSH_DEVICE_DOMAIN fallback also failed: %lld\n",
> > +		     status);
> > +	}
> > +
> > +	local_irq_restore(flags);
> > +}
> > +
> >  static void hv_iommu_iotlb_sync(struct iommu_domain *domain,
> >  				struct iommu_iotlb_gather *iotlb_gather)
> >  {
> > -	hv_flush_device_domain(to_hv_iommu_domain(domain));
> > +	hv_flush_device_domain_list(to_hv_iommu_domain(domain), iotlb_gather);
> > 
> >  	iommu_put_pages_list(&iotlb_gather->freelist);
> >  }
> > @@ -455,6 +520,7 @@ static struct iommu_domain *hv_iommu_domain_alloc_paging(struct device *dev)
> > 
> >  	cfg.common.hw_max_vasz_lg2 = hv_iommu_device->max_iova_width;
> >  	cfg.common.hw_max_oasz_lg2 = 52;
> > +	cfg.common.features |= BIT(PT_FEAT_FLUSH_RANGE);
> >  	cfg.top_level = (hv_iommu_device->max_iova_width > 48) ? 4 : 3;
> > 
> >  	ret = pt_iommu_x86_64_init(&hv_domain->pt_iommu_x86_64, &cfg, GFP_KERNEL);
> > diff --git a/include/hyperv/hvgdk_mini.h b/include/hyperv/hvgdk_mini.h
> > index 5bdbb44da112..eaaf87171478 100644
> > --- a/include/hyperv/hvgdk_mini.h
> > +++ b/include/hyperv/hvgdk_mini.h
> > @@ -496,6 +496,7 @@ union hv_vp_assist_msr_contents {	 /*
> > HV_REGISTER_VP_ASSIST_PAGE */
> >  #define HVCALL_GET_GPA_PAGES_ACCESS_STATES		0x00c9
> >  #define HVCALL_CONFIGURE_DEVICE_DOMAIN			0x00ce
> >  #define HVCALL_FLUSH_DEVICE_DOMAIN			0x00d0
> > +#define HVCALL_FLUSH_DEVICE_DOMAIN_LIST			0x00d1
> >  #define HVCALL_ACQUIRE_SPARSE_SPA_PAGE_HOST_ACCESS	0x00d7
> >  #define HVCALL_RELEASE_SPARSE_SPA_PAGE_HOST_ACCESS	0x00d8
> >  #define HVCALL_MODIFY_SPARSE_GPA_PAGE_HOST_VISIBILITY	0x00db
> > diff --git a/include/hyperv/hvhdk_mini.h b/include/hyperv/hvhdk_mini.h
> > index 493608e791b4..f51d5d9467f1 100644
> > --- a/include/hyperv/hvhdk_mini.h
> > +++ b/include/hyperv/hvhdk_mini.h
> > @@ -671,4 +671,21 @@ struct hv_input_flush_device_domain {
> >  	u32 reserved;
> >  } __packed;
> > 
> > +union hv_iommu_flush_va {
> > +	u64 iova;
> > +	struct {
> > +		u64 page_mask_shift : 12;
> > +		u64 page_number : 52;
> > +	};
> > +} __packed;
> > +
> > +
> > +struct hv_input_flush_device_domain_list {
> > +	struct hv_input_device_domain device_domain;
> > +#define HV_FLUSH_DEVICE_DOMAIN_LIST_IOMMU_FORMAT (1 << 0)
> 
> Use BIT()?
> 

Sure. 

> > +	u32 flags;
> > +	u32 reserved;
> > +	union hv_iommu_flush_va iova_list[];
> > +} __packed;
> > +
> >  #endif /* _HV_HVHDK_MINI_H */
> > --
> > 2.52.0
> > 
> 

B.R.
Yu