linux-kernel - RE: [PATCH v4 3/5] Drivers: hv: Batch GPA unmap operations to improve large region performance

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Message-ID:
 <SN6PR02MB41574B6F94BCC0E0F75DADFED4E0A@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Tue, 7 Oct 2025 00:02:14 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>
CC: "kys@...rosoft.com" <kys@...rosoft.com>, "haiyangz@...rosoft.com"
	<haiyangz@...rosoft.com>, "wei.liu@...nel.org" <wei.liu@...nel.org>,
	"decui@...rosoft.com" <decui@...rosoft.com>, "linux-hyperv@...r.kernel.org"
	<linux-hyperv@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v4 3/5] Drivers: hv: Batch GPA unmap operations to improve
 large region performance

From: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com> Sent: Monday, October 6, 2025 4:33 PM
> 
> On Mon, Oct 06, 2025 at 05:09:07PM +0000, Michael Kelley wrote:
> > From: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com> Sent: Monday, October 6, 2025 8:06 AM
> > >
> > > Reduce overhead when unmapping large memory regions by batching GPA unmap
> > > operations in 2MB-aligned chunks.
> > >
> > > Use a dedicated constant for batch size to improve code clarity and
> > > maintainability.
> > >
> > > Signed-off-by: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>
> > > ---
> > >  drivers/hv/mshv_root.h         |    2 ++
> > >  drivers/hv/mshv_root_hv_call.c |    2 +-
> > >  drivers/hv/mshv_root_main.c    |   28 +++++++++++++++++++++++++---
> > >  3 files changed, 28 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/drivers/hv/mshv_root.h b/drivers/hv/mshv_root.h
> > > index e3931b0f12693..97e64d5341b6e 100644
> > > --- a/drivers/hv/mshv_root.h
> > > +++ b/drivers/hv/mshv_root.h
> > > @@ -32,6 +32,8 @@ static_assert(HV_HYP_PAGE_SIZE == MSHV_HV_PAGE_SIZE);
> > >
> > >  #define MSHV_PIN_PAGES_BATCH_SIZE	(0x10000000ULL / HV_HYP_PAGE_SIZE)
> > >
> > > +#define MSHV_MAX_UNMAP_GPA_PAGES	512
> > > +
> > >  struct mshv_vp {
> > >  	u32 vp_index;
> > >  	struct mshv_partition *vp_partition;
> > > diff --git a/drivers/hv/mshv_root_hv_call.c b/drivers/hv/mshv_root_hv_call.c
> > > index c9c274f29c3c6..0696024ccfe31 100644
> > > --- a/drivers/hv/mshv_root_hv_call.c
> > > +++ b/drivers/hv/mshv_root_hv_call.c
> > > @@ -17,7 +17,7 @@
> > >  /* Determined empirically */
> > >  #define HV_INIT_PARTITION_DEPOSIT_PAGES 208
> > >  #define HV_MAP_GPA_DEPOSIT_PAGES	256
> > > -#define HV_UMAP_GPA_PAGES		512
> > > +#define HV_UMAP_GPA_PAGES		MSHV_MAX_UNMAP_GPA_PAGES
> > >
> > >  #define HV_PAGE_COUNT_2M_ALIGNED(pg_count) (!((pg_count) & (0x200 - 1)))
> > >
> > > diff --git a/drivers/hv/mshv_root_main.c b/drivers/hv/mshv_root_main.c
> > > index 97e322f3c6b5e..b61bef6b9c132 100644
> > > --- a/drivers/hv/mshv_root_main.c
> > > +++ b/drivers/hv/mshv_root_main.c
> > > @@ -1378,6 +1378,7 @@ mshv_map_user_memory(struct mshv_partition *partition,
> > >  static void mshv_partition_destroy_region(struct mshv_mem_region *region)
> > >  {
> > >  	struct mshv_partition *partition = region->partition;
> > > +	u64 gfn, gfn_count, start_gfn, end_gfn;
> > >  	u32 unmap_flags = 0;
> > >  	int ret;
> > >
> > > @@ -1396,9 +1397,30 @@ static void mshv_partition_destroy_region(struct mshv_mem_region *region)
> > >  	if (region->flags.large_pages)
> > >  		unmap_flags |= HV_UNMAP_GPA_LARGE_PAGE;
> > >
> > > -	/* ignore unmap failures and continue as process may be exiting */
> > > -	hv_call_unmap_gpa_pages(partition->pt_id, region->start_gfn,
> > > -				region->nr_pages, unmap_flags);
> > > +	start_gfn = region->start_gfn;
> > > +	end_gfn = region->start_gfn + region->nr_pages;
> > > +
> > > +	for (gfn = start_gfn; gfn < end_gfn; gfn += gfn_count) {
> > > +		if (gfn % MSHV_MAX_UNMAP_GPA_PAGES)
> > > +			gfn_count = ALIGN(gfn, MSHV_MAX_UNMAP_GPA_PAGES) - gfn;
> > > +		else
> > > +			gfn_count = MSHV_MAX_UNMAP_GPA_PAGES;
> >
> > You could do the entire if/else as:
> >
> > 		gfn_count = ALIGN(gfn + 1, MSHV_MAX_UNMAP_GPA_PAGES) - gfn;
> >
> > Using "gfn + 1" handles the case where gfn is already aligned. Arguably, this is a bit
> > more obscure, so it's just a suggestion.
> >
> > > +
> > > +		if (gfn + gfn_count > end_gfn)
> > > +			gfn_count = end_gfn - gfn;
> >
> > Or
> > 		gfn_count = min(gfn_count, end_gfn - gfn);
> >
> > I usually prefer the "min" function instead of an "if" statement if logically
> > the intent is to compute the minimum. But again, just a suggestion.
> >
> > > +
> > > +		/* Skip if all pages in this range if none is mapped */
> > > +		if (!memchr_inv(region->pages + (gfn - start_gfn), 0,
> > > +				gfn_count * sizeof(struct page *)))
> > > +			continue;
> > > +
> > > +		ret = hv_call_unmap_gpa_pages(partition->pt_id, gfn,
> > > +					      gfn_count, unmap_flags);
> > > +		if (ret)
> > > +			pt_err(partition,
> > > +			       "Failed to unmap GPA pages %#llx-%#llx: %d\n",
> > > +			       gfn, gfn + gfn_count - 1, ret);
> > > +	}
> >
> > Overall, I think this algorithm looks good and handles all the edge cases.
> >
> 
> Thank you for your suggestions. I also generally prefer reducing the
> code in a similar way, but in this case, I deliberately chose a more
> elaborate approach to improve clarity.
> 
> So, if you don’t mind, I’d rather keep it as is, since this version is
> easy to understand and self-documenting.
> 

Yes, that's fine with me.

Reviewed-by: Michael Kelley <mhklinux@...look.com>