linux-kernel - RE: [PATCH v2 2/2] mshv: Allow mappings that overlap in uaddr

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Message-ID:
 <SN6PR02MB41571611B46C233A6CFC86E4D4C3A@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Fri, 7 Nov 2025 15:25:12 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Nuno Das Neves <nunodasneves@...ux.microsoft.com>,
	"linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"magnuskulke@...ux.microsoft.com" <magnuskulke@...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>, "longli@...rosoft.com"
	<longli@...rosoft.com>, "skinsburskii@...ux.microsoft.com"
	<skinsburskii@...ux.microsoft.com>, "prapal@...ux.microsoft.com"
	<prapal@...ux.microsoft.com>, "mrathor@...ux.microsoft.com"
	<mrathor@...ux.microsoft.com>, "muislam@...rosoft.com"
	<muislam@...rosoft.com>
Subject: RE: [PATCH v2 2/2] mshv: Allow mappings that overlap in uaddr

From: Nuno Das Neves <nunodasneves@...ux.microsoft.com> Sent: Thursday, November 6, 2025 2:14 PM
> 
> From: Magnus Kulke <magnuskulke@...ux.microsoft.com>
> 
> Currently the MSHV driver rejects mappings that would overlap in
> userspace.
> 
> Some VMMs require the same memory to be mapped to different parts of
> the guest's address space, and so working around this restriction is
> difficult.
> 
> The hypervisor itself doesn't prohibit mappings that overlap in uaddr,
> (really in SPA; system physical addresses), so supporting this in the
> driver doesn't require any extra work: only the checks need to be
> removed.
> 
> Since no userspace code until now has been able to overlap regions in
> userspace, relaxing this constraint can't break any existing code.
> 
> Signed-off-by: Magnus Kulke <magnuskulke@...ux.microsoft.com>
> Signed-off-by: Nuno Das Neves <nunodasneves@...ux.microsoft.com>
> ---
>  drivers/hv/mshv_root_main.c | 8 ++------
>  include/uapi/linux/mshv.h   | 2 +-
>  2 files changed, 3 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/hv/mshv_root_main.c b/drivers/hv/mshv_root_main.c
> index 25a68912a78d..b1821b18fa09 100644
> --- a/drivers/hv/mshv_root_main.c
> +++ b/drivers/hv/mshv_root_main.c
> @@ -1220,12 +1220,8 @@ static int mshv_partition_create_region(struct mshv_partition *partition,
> 
>  	/* Reject overlapping regions */
>  	hlist_for_each_entry(rg, &partition->pt_mem_regions, hnode) {
> -		u64 rg_size = rg->nr_pages << HV_HYP_PAGE_SHIFT;
> -
> -		if ((mem->guest_pfn + nr_pages <= rg->start_gfn ||
> -		     rg->start_gfn + rg->nr_pages <= mem->guest_pfn) &&
> -		    (mem->userspace_addr + mem->size <= rg->start_uaddr ||
> -		     rg->start_uaddr + rg_size <= mem->userspace_addr))
> +		if (mem->guest_pfn + nr_pages <= rg->start_gfn ||
> +		    rg->start_gfn + rg->nr_pages <= mem->guest_pfn)
>  			continue;
> 
>  		return -EEXIST;
> diff --git a/include/uapi/linux/mshv.h b/include/uapi/linux/mshv.h
> index 9091946cba23..b10c8d1cb2ad 100644
> --- a/include/uapi/linux/mshv.h
> +++ b/include/uapi/linux/mshv.h
> @@ -123,7 +123,7 @@ enum {
>   * @rsvd: MBZ
>   *
>   * Map or unmap a region of userspace memory to Guest Physical Addresses (GPA).
> - * Mappings can't overlap in GPA space or userspace.
> + * Mappings can't overlap in GPA space.
>   * To unmap, these fields must match an existing mapping.
>   */
>  struct mshv_user_mem_region {
> --
> 2.34.1

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