linux-kernel - Re: [PATCH 4/5] vfio/iommu_type1: Carefully use unmap_unpin

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Message-ID: <20210112125331.789f47a5@omen.home.shazbot.org>
Date:   Tue, 12 Jan 2021 12:53:31 -0700
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Keqian Zhu <zhukeqian1@...wei.com>
Cc:     <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <iommu@...ts.linux-foundation.org>, <kvm@...r.kernel.org>,
        <kvmarm@...ts.cs.columbia.edu>,
        Kirti Wankhede <kwankhede@...dia.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Will Deacon <will@...nel.org>, "Marc Zyngier" <maz@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        "Mark Rutland" <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        "Robin Murphy" <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Suzuki K Poulose" <suzuki.poulose@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexios Zavras <alexios.zavras@...el.com>,
        <wanghaibin.wang@...wei.com>, <jiangkunkun@...wei.com>
Subject: Re: [PATCH 4/5] vfio/iommu_type1: Carefully use unmap_unpin_all
 during dirty tracking

On Tue, 12 Jan 2021 20:04:38 +0800
Keqian Zhu <zhukeqian1@...wei.com> wrote:

> On 2021/1/12 5:49, Alex Williamson wrote:
> > On Thu, 7 Jan 2021 17:29:00 +0800
> > Keqian Zhu <zhukeqian1@...wei.com> wrote:
> >   
> >> If we detach group during dirty page tracking, we shouldn't remove
> >> vfio_dma, because dirty log will lose.
> >>
> >> But we don't prevent unmap_unpin_all in vfio_iommu_release, because
> >> under normal procedure, dirty tracking has been stopped.  
> > 
> > This looks like it's creating a larger problem than it's fixing, it's
> > not our job to maintain the dirty bitmap regardless of what the user
> > does.  If the user detaches the last group in a container causing the
> > mappings within that container to be deconstructed before the user has
> > collected dirty pages, that sounds like a user error.  A container with
> > no groups is de-privileged and therefore loses all state.  Thanks,
> > 
> > Alex  
> 
> Hi Alex,
> 
> This looks good to me ;-). That's a reasonable constraint for user behavior.
> 
> What about replacing this patch with an addition to the uapi document of
> VFIO_GROUP_UNSET_CONTAINER? User should pay attention to this when call this
> ioctl during dirty tracking.

Here's the current uapi comment:

/**
 * VFIO_GROUP_UNSET_CONTAINER - _IO(VFIO_TYPE, VFIO_BASE + 5)
 *
 * Remove the group from the attached container.  This is the
 * opposite of the SET_CONTAINER call and returns the group to
 * an initial state.  All device file descriptors must be released
 * prior to calling this interface.  When removing the last group
 * from a container, the IOMMU will be disabled and all state lost,
 * effectively also returning the VFIO file descriptor to an initial
 * state.
 * Return: 0 on success, -errno on failure.
 * Availability: When attached to container
 */

So we already indicate that "all state" of the container is lost when
removing the last group, I don't see that it's necessarily to
explicitly include dirty bitmap state beyond that statement.  Without
mappings there can be no dirty bitmap to track.

 > And any comments on other patches? thanks.

I had a difficult time mapping the commit log to the actual code
change, I'll likely have some wording suggestions.  Is patch 5/5 still
necessary if this patch is dropped?  Thanks,

Alex

> >> Fixes: d6a4c185660c ("vfio iommu: Implementation of ioctl for dirty pages tracking")
> >> Signed-off-by: Keqian Zhu <zhukeqian1@...wei.com>
> >> ---
> >>  drivers/vfio/vfio_iommu_type1.c | 14 ++++++++++++--
> >>  1 file changed, 12 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> >> index 26b7eb2a5cfc..9776a059904d 100644
> >> --- a/drivers/vfio/vfio_iommu_type1.c
> >> +++ b/drivers/vfio/vfio_iommu_type1.c
> >> @@ -2373,7 +2373,12 @@ static void vfio_iommu_type1_detach_group(void *iommu_data,
> >>  			if (list_empty(&iommu->external_domain->group_list)) {
> >>  				vfio_sanity_check_pfn_list(iommu);
> >>  
> >> -				if (!IS_IOMMU_CAP_DOMAIN_IN_CONTAINER(iommu))
> >> +				/*
> >> +				 * During dirty page tracking, we can't remove
> >> +				 * vfio_dma because dirty log will lose.
> >> +				 */
> >> +				if (!IS_IOMMU_CAP_DOMAIN_IN_CONTAINER(iommu) &&
> >> +				    !iommu->dirty_page_tracking)
> >>  					vfio_iommu_unmap_unpin_all(iommu);
> >>  
> >>  				kfree(iommu->external_domain);
> >> @@ -2406,10 +2411,15 @@ static void vfio_iommu_type1_detach_group(void *iommu_data,
> >>  		 * iommu and external domain doesn't exist, then all the
> >>  		 * mappings go away too. If it's the last domain with iommu and
> >>  		 * external domain exist, update accounting
> >> +		 *
> >> +		 * Note: During dirty page tracking, we can't remove vfio_dma
> >> +		 * because dirty log will lose. Just update accounting is a good
> >> +		 * choice.
> >>  		 */
> >>  		if (list_empty(&domain->group_list)) {
> >>  			if (list_is_singular(&iommu->domain_list)) {
> >> -				if (!iommu->external_domain)
> >> +				if (!iommu->external_domain &&
> >> +				    !iommu->dirty_page_tracking)
> >>  					vfio_iommu_unmap_unpin_all(iommu);
> >>  				else
> >>  					vfio_iommu_unmap_unpin_reaccount(iommu);  
> > 
> > .
> >   
>