linux-kernel - RE: [PATCH 3/5] vfio/iommu_type1: Prefer to reuse domains vs match enforced cache coherency

Open Source and information security mailing list archives

Date:   Wed, 8 Jun 2022 08:28:03 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Nicolin Chen <nicolinc@...dia.com>,
        "jgg@...dia.com" <jgg@...dia.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "will@...nel.org" <will@...nel.org>,
        "marcan@...can.st" <marcan@...can.st>,
        "sven@...npeter.dev" <sven@...npeter.dev>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "robdclark@...il.com" <robdclark@...il.com>,
        "m.szyprowski@...sung.com" <m.szyprowski@...sung.com>,
        "krzysztof.kozlowski@...aro.org" <krzysztof.kozlowski@...aro.org>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "agross@...nel.org" <agross@...nel.org>,
        "bjorn.andersson@...aro.org" <bjorn.andersson@...aro.org>,
        "matthias.bgg@...il.com" <matthias.bgg@...il.com>,
        "heiko@...ech.de" <heiko@...ech.de>,
        "orsonzhai@...il.com" <orsonzhai@...il.com>,
        "baolin.wang7@...il.com" <baolin.wang7@...il.com>,
        "zhang.lyra@...il.com" <zhang.lyra@...il.com>,
        "wens@...e.org" <wens@...e.org>,
        "jernej.skrabec@...il.com" <jernej.skrabec@...il.com>,
        "samuel@...lland.org" <samuel@...lland.org>,
        "jean-philippe@...aro.org" <jean-philippe@...aro.org>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>
CC:     "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "thierry.reding@...il.com" <thierry.reding@...il.com>,
        "alim.akhtar@...sung.com" <alim.akhtar@...sung.com>,
        "alyssa@...enzweig.io" <alyssa@...enzweig.io>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "linux-samsung-soc@...r.kernel.org" 
        <linux-samsung-soc@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "jonathanh@...dia.com" <jonathanh@...dia.com>,
        "linux-rockchip@...ts.infradead.org" 
        <linux-rockchip@...ts.infradead.org>,
        "gerald.schaefer@...ux.ibm.com" <gerald.schaefer@...ux.ibm.com>,
        "linux-sunxi@...ts.linux.dev" <linux-sunxi@...ts.linux.dev>,
        "linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
        "linux-mediatek@...ts.infradead.org" 
        <linux-mediatek@...ts.infradead.org>,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "cohuck@...hat.com" <cohuck@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>
Subject: RE: [PATCH 3/5] vfio/iommu_type1: Prefer to reuse domains vs match
 enforced cache coherency

> From: Nicolin Chen
> Sent: Monday, June 6, 2022 2:19 PM
> 
> From: Jason Gunthorpe <jgg@...dia.com>
> 
> The KVM mechanism for controlling wbinvd is only triggered during
> kvm_vfio_group_add(), meaning it is a one-shot test done once the devices
> are setup.

It's not one-shot. kvm_vfio_update_coherency() is called in both
group_add() and group_del(). Then the coherency property is
checked dynamically in wbinvd emulation:

kvm_emulate_wbinvd()
  kvm_emulate_wbinvd_noskip()
    need_emulate_wbinvd()
      kvm_arch_has_noncoherent_dma()

It's also checked when a vcpu is scheduled to a new cpu for
tracking dirty cpus which requires cache flush when emulating
wbinvd on that vcpu. See kvm_arch_vcpu_load().

	/* Address WBINVD may be executed by guest */
	if (need_emulate_wbinvd(vcpu)) {
		if (static_call(kvm_x86_has_wbinvd_exit)())
			cpumask_set_cpu(cpu, vcpu->arch.wbinvd_dirty_mask);

In addition, it's also checked when deciding the effective memory
type of EPT entry. See vmx_get_mt_mask().

	if (!kvm_arch_has_noncoherent_dma(vcpu->kvm))
		return (MTRR_TYPE_WRBACK << VMX_EPT_MT_EPTE_SHIFT) | VMX_EPT_IPAT_BIT;

But I doubt above can work reliably when the property is changed
in the fly given above paths are triggered at different points. The
guest may end up in a mixed state where inconsistent coherency 
is assumed in different emulation paths.

and In reality I don't think such niche scenario is even tested 
given the only device imposing such trick is integrated Intel GPU
which iiuc no one would try to hotplug/hot-remove it to/from
a guest.

given that I'm fine with the change in this patch. Even more probably
we really want an explicit one-shot model so KVM can lock down
the property once it starts to consume it then further adding a new
group which would change the coherency is explicitly rejected and
removing an existing group leaves it intact.

> 
> So, there is no value in trying to push a device that could do enforced
> cache coherency to a dedicated domain vs re-using an existing domain since

"an existing domain (even if it doesn't enforce coherency)", otherwise if
it's already compatible there is no question here.

> KVM won't be able to take advantage of it. This just wastes domain memory.
> 
> Simplify this code and eliminate the test. This removes the only logic
> that needed to have a dummy domain attached prior to searching for a
> matching domain and simplifies the next patches.
> 
> If someday we want to try and optimize this further the better approach is
> to update the Intel driver so that enforce_cache_coherency() can work on a
> domain that already has IOPTEs and then call the enforce_cache_coherency()
> after detaching a device from a domain to upgrade the whole domain to
> enforced cache coherency mode.
> 
> Signed-off-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: Nicolin Chen <nicolinc@...dia.com>
> ---
>  drivers/vfio/vfio_iommu_type1.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/drivers/vfio/vfio_iommu_type1.c
> b/drivers/vfio/vfio_iommu_type1.c
> index c13b9290e357..f4e3b423a453 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -2285,9 +2285,7 @@ static int vfio_iommu_type1_attach_group(void
> *iommu_data,
>  	 * testing if they're on the same bus_type.
>  	 */
>  	list_for_each_entry(d, &iommu->domain_list, next) {
> -		if (d->domain->ops == domain->domain->ops &&
> -		    d->enforce_cache_coherency ==
> -			    domain->enforce_cache_coherency) {
> +		if (d->domain->ops == domain->domain->ops) {
>  			iommu_detach_group(domain->domain, group-
> >iommu_group);
>  			if (!iommu_attach_group(d->domain,
>  						group->iommu_group)) {
> --
> 2.17.1
> 
> _______________________________________________
> iommu mailing list
> iommu@...ts.linux-foundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/iommu

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives