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

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Message-ID:
 <SN6PR02MB4157F402AB89602C08A6109FD4F6A@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Fri, 17 Oct 2025 18:09:42 +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>
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>, "arnd@...db.de" <arnd@...db.de>,
	"mrathor@...ux.microsoft.com" <mrathor@...ux.microsoft.com>,
	"skinsburskii@...ux.microsoft.com" <skinsburskii@...ux.microsoft.com>
Subject: RE: [PATCH] mshv: Fix deposit memory in MSHV_ROOT_HVCALL

From: Nuno Das Neves <nunodasneves@...ux.microsoft.com> Sent: Friday, October 17, 2025 10:55 AM
> 
> On 10/17/2025 10:33 AM, Michael Kelley wrote:
> > From: Nuno Das Neves <nunodasneves@...ux.microsoft.com> Sent: Friday, October 17, 2025 10:26 AM
> >>
> >> On 10/16/2025 6:12 PM, Michael Kelley wrote:
> >>> From: Nuno Das Neves <nunodasneves@...ux.microsoft.com> Sent: Thursday, October 16, 2025 12:54 PM
> >>>>
> >>>> 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.
> >>>>
> >>>> However, it's much easier and efficient if the driver simply deposits
> >>>> memory on demand and immediately retries the hypercall as is done with
> >>>> all the other hypercall helper functions.
> >>>>
> >>>> But unlike those, 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.
> >>>
> >>> >From reading the above, I'm pretty sure this code change is an
> >>> optimization that lets user space avoid having to deal with the
> >>> -EAGAIN result by resubmitting the ioctl with a different
> >>> starting point for a rep hypercall. As such, I'd suggest the patch
> >>> title should be "Improve deposit memory ...." (or something similar).
> >>> The word "Fix" makes it sound like a bug fix.
> >>>
> >>> Or is user space code currently faulty in its handling of -EAGAIN, and
> >>> this really is an indirect bug fix to make things work? If so, do you
> >>> want a Fixes: tag so the change is backported?
> >>>
> >>
> >> It's the latter case, userspace doesn't handle it correctly, so I
> >> consider it a fix more than just an improvement.
> >
> > OK, thanks. You might want to tweak the commit message a bit
> > to clarify that this really is a bug fix and why. I was leaning
> > toward the wrong conclusion based on the current commit
> > message.
> >
> 
> Is this clearer?
> 
> "
> mshv: Fix deposit memory in MSHV_ROOT_HVCALL
> 
> 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.

Yes, that works for me.  Thanks.

Michael