linux-kernel - RE: [PATCH v6 3/5] Drivers: hv: Batch GPA unmap operations to improve large region performance

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Message-ID:
 <SN6PR02MB41574B72041F6AFB1B83DE36D4D6A@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Tue, 18 Nov 2025 16:28:55 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>,
	"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>
CC: "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v6 3/5] Drivers: hv: Batch GPA unmap operations to improve
 large region performance

From: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com> Sent: Monday, November 17, 2025 8:53 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>
> Reviewed-by: Michael Kelley <mhklinux@...look.com>
> Reviewed-by: Nuno Das Neves <nunodasneves@...ux.microsoft.com>
> ---
>  drivers/hv/mshv_root.h         |    2 ++
>  drivers/hv/mshv_root_hv_call.c |    2 +-
>  drivers/hv/mshv_root_main.c    |   36 ++++++++++++++++++++++++++++++------
>  3 files changed, 33 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/hv/mshv_root.h b/drivers/hv/mshv_root.h
> index 3eb815011b46..5eece7077f8b 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 caf02cfa49c9..8fa983f1109b 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 a85872b72b1a..ef36d8115d8a 100644
> --- a/drivers/hv/mshv_root_main.c
> +++ b/drivers/hv/mshv_root_main.c
> @@ -1365,7 +1365,7 @@ 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;
> -	u32 unmap_flags = 0;
> +	u64 gfn, gfn_count, start_gfn, end_gfn;
>  	int ret;
> 
>  	hlist_del(&region->hnode);
> @@ -1380,12 +1380,36 @@ static void mshv_partition_destroy_region(struct mshv_mem_region *region)
>  		}
>  	}
> 
> -	if (region->flags.large_pages)
> -		unmap_flags |= HV_UNMAP_GPA_LARGE_PAGE;
> +	start_gfn = region->start_gfn;
> +	end_gfn = region->start_gfn + region->nr_pages;
> +
> +	for (gfn = start_gfn; gfn < end_gfn; gfn += gfn_count) {
> +		u32 unmap_flags = 0;
> +
> +		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;
> +
> +		if (gfn + gfn_count > end_gfn)
> +			gfn_count = end_gfn - gfn;
> 
> -	/* 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);
> +		/* Skip all pages in this range if none are mapped */
> +		if (!memchr_inv(region->pages + (gfn - start_gfn), 0,
> +				gfn_count * sizeof(struct page *)))
> +			continue;
> +
> +		if (region->flags.large_pages &&
> +		    VALUE_PMD_ALIGNED(gfn) && VALUE_PMD_ALIGNED(gfn_count))

VALUE_PMD_ALIGNED isn't defined until Patch 4 of this series.

The idea of a page count being PMD aligned occurs a few other places in
Linux kernel code, and it is usually written as IS_ALIGNED(count, PTRS_PER_PMD),
though there's one occurrence of !(count % PTRS_PER_PMD).

Also mshv_root_hv_call.c has HV_PAGE_COUNT_2M_ALIGNED() that does
the same thing. Some macro consolidation is appropriate, or just open code
as IS_ALIGNED(<cnt>, PTRS_PER_PMD) and eliminate the macros.

> +			unmap_flags |= HV_UNMAP_GPA_LARGE_PAGE;
> +
> +		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);
> +	}
> 
>  	mshv_region_invalidate(region);
> 
> 
>