linux-kernel - Re: [PATCH v13 4/6] iommu/vt-d: Add support for IOMMU default DMA mode build options

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

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <f3fe6c4b-f360-ab7b-7ad2-ced63269499d@huawei.com>
Date:   Thu, 17 Jun 2021 09:00:37 +0100
From:   John Garry <john.garry@...wei.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>, <joro@...tes.org>,
        <will@...nel.org>, <dwmw2@...radead.org>, <robin.murphy@....com>,
        <corbet@....net>
CC:     <linux-kernel@...r.kernel.org>, <iommu@...ts.linux-foundation.org>,
        <linuxarm@...wei.com>, <thunder.leizhen@...wei.com>,
        <chenxiang66@...ilicon.com>, <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v13 4/6] iommu/vt-d: Add support for IOMMU default DMA
 mode build options

On 17/06/2021 08:32, Lu Baolu wrote:
> On 6/16/21 7:03 PM, John Garry wrote:
>> @@ -4382,9 +4380,9 @@ int __init intel_iommu_init(void)
>>            * is likely to be much lower than the overhead of 
>> synchronizing
>>            * the virtual and physical IOMMU page-tables.
>>            */
>> -        if (!intel_iommu_strict && cap_caching_mode(iommu->cap)) {
>> -            pr_warn("IOMMU batching is disabled due to virtualization");
>> -            intel_iommu_strict = 1;
>> +        if (cap_caching_mode(iommu->cap)) {
>> +            pr_warn("IOMMU batching disallowed due to 
>> virtualization\n");
>> +            iommu_set_dma_strict(true);
> 
> With this change, VM guest will always show this warning.

Would they have got it before also normally?

I mean, default is intel_iommu_strict=0, so if 
cap_caching_mode(iommu->cap) is true and intel_iommu_strict not set to 1 
elsewhere previously, then we would get this print.

> How about
> removing this message? Users could get the same information through the
> kernel message added by "[PATCH v13 2/6] iommu: Print strict or lazy
> mode at init time".

I think that the print from 2/6 should occur before this print.

Regardless I would think that you would still like to be notified of 
this change in policy, right?

However I now realize that the print is in a loop per iommu, so we would 
get it per iommu:

for_each_active_iommu(iommu, drhd) {
	/*
	 * The flush queue implementation does not perform
	 * page-selective invalidations that are required for efficient
	 * TLB flushes in virtual environments.  The benefit of batching
	 * is likely to be much lower than the overhead of synchronizing
	 * the virtual and physical IOMMU page-tables.
	 */
	if (!intel_iommu_strict && cap_caching_mode(iommu->cap)) {
		pr_warn("IOMMU batching is disabled due to virtualization");
		intel_iommu_strict = 1;
	}
	...
}

I need to change that. How about this:

bool print_warning = false;

for_each_active_iommu(iommu, drhd) {
	/*
	 * The flush queue implementation does not perform
	 * page-selective invalidations that are required for efficient
	 * TLB flushes in virtual environments.  The benefit of batching
	 * is likely to be much lower than the overhead of synchronizing
	 * the virtual and physical IOMMU page-tables.
	 */
	if (!print_warning && cap_caching_mode(iommu->cap)) {
		pr_warn("IOMMU batching disallowed due to virtualization\n");
		iommu_set_dma_strict(true);
		print_warning = true;
	}
	...
}

or use pr_warn_once().

Thanks,
John