From: Yu Zhang <zhangyu1@linux.microsoft.com>
To: Jason Gunthorpe <jgg@ziepe.ca>, Michael Kelley <mhklinux@outlook.com>
Cc: linux-kernel@vger.kernel.org, linux-hyperv@vger.kernel.org,
iommu@lists.linux.dev, linux-pci@vger.kernel.org,
linux-arch@vger.kernel.org, wei.liu@kernel.org,
kys@microsoft.com, haiyangz@microsoft.com, decui@microsoft.com,
longli@microsoft.com, joro@8bytes.org, will@kernel.org,
robin.murphy@arm.com, bhelgaas@google.com,
kwilczynski@kernel.org, lpieralisi@kernel.org, mani@kernel.org,
robh@kernel.org, arnd@arndb.de, jacob.pan@linux.microsoft.com,
tgopinath@linux.microsoft.com,
easwar.hariharan@linux.microsoft.com
Subject: Re: [PATCH v1 4/4] iommu/hyperv: Add page-selective IOTLB flush support
Date: Thu, 21 May 2026 01:15:24 +0800 [thread overview]
Message-ID: <lxmfd2ml5dafkxquuf5i45uqgh6wxl44hlqphu77kvximqrnmi@b3htoyjc6d5z> (raw)
In-Reply-To: <20260515223545.GL7702@ziepe.ca>
On Fri, May 15, 2026 at 07:35:45PM -0300, Jason Gunthorpe wrote:
> On Tue, May 12, 2026 at 12:24:08AM +0800, Yu Zhang wrote:
> > +static inline u16 hv_iommu_fill_iova_list(union hv_iommu_flush_va *iova_list,
> > + unsigned long start,
> > + unsigned long end)
> > +{
> > + unsigned long start_pfn = start >> PAGE_SHIFT;
> > + unsigned long end_pfn = PAGE_ALIGN(end) >> PAGE_SHIFT;
> > + unsigned long nr_pages = end_pfn - start_pfn;
> > + u16 count = 0;
> > +
> > + while (nr_pages > 0) {
> > + unsigned long flush_pages;
> > + int order;
> > + unsigned long pfn_align;
> > + unsigned long size_align;
> > +
> > + if (count >= HV_IOMMU_MAX_FLUSH_VA_COUNT) {
> > + count = HV_IOMMU_FLUSH_VA_OVERFLOW;
> > + break;
> > + }
> > +
> > + if (start_pfn)
> > + pfn_align = __ffs(start_pfn);
> > + else
> > + pfn_align = BITS_PER_LONG - 1;
> > +
> > + size_align = __fls(nr_pages);
> > + order = min(pfn_align, size_align);
> > + iova_list[count].page_mask_shift = order;
> > + iova_list[count].page_number = start_pfn;
> > +
> > + flush_pages = 1UL << order;
> > + start_pfn += flush_pages;
> > + nr_pages -= flush_pages;
> > + count++;
> > + }
>
> This seems like a really silly hypervisor interface. Why doesn't it
> just accept a normal range? Splitting it into power of two aligned
> ranges is very inefficient.
Fair point. I'm not sure how much flexibility we have to change
this hypercall interface at the moment - it predates the pvIOMMU
work and may have other consumers beyond Linux guest. On the other
hand, having the guest specify 2^N-aligned blocks does save the
hypervisor from having to decompose ranges itself before issuing
hardware invalidation commands - the guest-provided entries can be
fed to the HW more or less directly.
That said, the way I'm currently using this interface may be
more precise than necessary. Maybe we have 2 options:
1) Current approach: decompose the range into multiple exact
2^N-aligned blocks with no over-flush, but at the cost of
more complex calculations and more entries.
2) Follow what Intel/AMD drivers do: find a single minimal
2^N-aligned block that covers the entire range, but may
over-flush.
Any preference?
@Michael, since you've also been reviewing this patch, I'd
appreciate your thoughts on the above as well. :)
Yu
next prev parent reply other threads:[~2026-05-20 17:15 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-05-11 16:24 [PATCH v1 0/4] Hyper-V: Add para-virtualized IOMMU support for Linux guests Yu Zhang
2026-05-11 16:24 ` [PATCH v1 1/4] iommu: Move Hyper-V IOMMU driver to its own subdirectory Yu Zhang
2026-05-15 22:19 ` Jason Gunthorpe
2026-05-20 6:37 ` Yu Zhang
2026-05-20 13:38 ` Jason Gunthorpe
2026-05-11 16:24 ` [PATCH v1 2/4] hyperv: Introduce new hypercall interfaces used by Hyper-V guest IOMMU Yu Zhang
2026-05-12 21:24 ` sashiko-bot
2026-05-11 16:24 ` [PATCH v1 3/4] iommu/hyperv: Add para-virtualized IOMMU support for Hyper-V guest Yu Zhang
2026-05-12 22:30 ` sashiko-bot
2026-05-13 18:39 ` Jacob Pan
2026-05-15 12:38 ` Yu Zhang
2026-05-14 18:13 ` Michael Kelley
2026-05-15 13:59 ` Yu Zhang
2026-05-15 14:51 ` Michael Kelley
2026-05-15 16:53 ` Yu Zhang
2026-05-15 17:36 ` Michael Kelley
2026-05-20 15:50 ` Yu Zhang
2026-05-16 0:11 ` Mukesh R
2026-05-18 9:38 ` Yu Zhang
2026-05-15 22:31 ` Jason Gunthorpe
2026-05-20 15:25 ` Yu Zhang
2026-05-20 18:27 ` Jacob Pan
2026-05-21 12:27 ` Yu Zhang
2026-05-11 16:24 ` [PATCH v1 4/4] iommu/hyperv: Add page-selective IOTLB flush support Yu Zhang
2026-05-12 23:45 ` sashiko-bot
2026-05-14 18:14 ` Michael Kelley
2026-05-14 21:16 ` Michael Kelley
2026-05-15 16:23 ` Yu Zhang
2026-05-15 18:00 ` Michael Kelley
2026-05-15 23:33 ` Michael Kelley
2026-05-20 16:27 ` Yu Zhang
2026-05-15 22:35 ` Jason Gunthorpe
2026-05-20 17:15 ` Yu Zhang [this message]
2026-05-20 17:29 ` Jason Gunthorpe
2026-05-20 19:26 ` Michael Kelley
2026-05-20 20:40 ` Jacob Pan
2026-05-21 15:45 ` Michael Kelley
2026-05-21 14:34 ` Yu Zhang
2026-05-21 15:39 ` Michael Kelley
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