linux-kernel - Re: [PATCH v2] mshv: Fix deposit memory in MSHV_ROOT

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Message-ID: <20251017222633.GA632885@liuwe-devbox-debian-v2.local>
Date: Fri, 17 Oct 2025 22:26:33 +0000
From: Wei Liu <wei.liu@...nel.org>
To: Nuno Das Neves <nunodasneves@...ux.microsoft.com>
Cc: linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
	mhklinux@...look.com, kys@...rosoft.com, haiyangz@...rosoft.com,
	wei.liu@...nel.org, decui@...rosoft.com, arnd@...db.de,
	mrathor@...ux.microsoft.com, skinsburskii@...ux.microsoft.com
Subject: Re: [PATCH v2] mshv: Fix deposit memory in MSHV_ROOT_HVCALL

On Fri, Oct 17, 2025 at 10:06:55PM +0000, Wei Liu wrote:
> On Fri, Oct 17, 2025 at 11:58:17AM -0700, Nuno Das Neves wrote:
> > When the MSHV_ROOT_HVCALL ioctl is executing a hypercall, and gets
> > HV_STATUS_INSUFFICIENT_MEMORY, it deposits memory and then returns
> > -EAGAIN to userspace. The expectation is that the VMM will retry.
> > 
> > However, some VMM code in the wild doesn't do this and simply fails.
> > Rather than force the VMM to retry, change the ioctl to deposit
> > memory on demand and immediately retry the hypercall as is done with
> > all the other hypercall helper functions.
> > 
> > In addition to making the ioctl easier to use, removing the need for
> > multiple syscalls improves performance.
> > 
> > There is a complication: unlike the other hypercall helper functions,
> > in MSHV_ROOT_HVCALL the input is opaque to the kernel. This is
> > problematic for rep hypercalls, because the next part of the input
> > list can't be copied on each loop after depositing pages (this was
> > the original reason for returning -EAGAIN in this case).
> > 
> > Introduce hv_do_rep_hypercall_ex(), which adds a 'rep_start'
> > parameter. This solves the issue, allowing the deposit loop in
> > MSHV_ROOT_HVCALL to restart a rep hypercall after depositing pages
> > partway through.
> > 
> > Signed-off-by: Nuno Das Neves <nunodasneves@...ux.microsoft.com>
> 
> In v1 you said you will add a "Fixes" tag. Where is it?

I added this:

Fixes: 621191d709b1 ("Drivers: hv: Introduce mshv_root module to expose /dev/mshv to VMMs")

Let me know if that's not correct.

Wei