linux-kernel - Re: [PATCH v2 1/1] iommu/vt-d: Serialize IOMMU GCMD register modifications

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Date:   Fri, 28 Aug 2020 08:00:06 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Joerg Roedel <joro@...tes.org>
Cc:     baolu.lu@...ux.intel.com,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        "Raj, Ashok" <ashok.raj@...el.com>
Subject: Re: [PATCH v2 1/1] iommu/vt-d: Serialize IOMMU GCMD register
 modifications

Hi,

On 8/27/20 1:39 PM, Tian, Kevin wrote:
>> From: Lu Baolu <baolu.lu@...ux.intel.com>
>> Sent: Thursday, August 27, 2020 12:25 PM
>>
>> The VT-d spec requires (10.4.4 Global Command Register, GCMD_REG
>> General
>> Description) that:
>>
>> If multiple control fields in this register need to be modified, software
>> must serialize the modifications through multiple writes to this register.
>>
>> However, in irq_remapping.c, modifications of IRE and CFI are done in one
>> write. We need to do two separate writes with STS checking after each.
>>
>> Fixes: af8d102f999a4 ("x86/intel/irq_remapping: Clean up x2apic opt-out
>> security warning mess")
>> Cc: Andy Lutomirski <luto@...capital.net>
>> Cc: Jacob Pan <jacob.jun.pan@...ux.intel.com>
>> Cc: Kevin Tian <kevin.tian@...el.com>
>> Cc: Ashok Raj <ashok.raj@...el.com>
>> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
>> ---
>>   drivers/iommu/intel/irq_remapping.c | 11 +++++++++--
>>   1 file changed, 9 insertions(+), 2 deletions(-)
>>
>> Change log:
>> v1->v2:
>>    - v1 posted here
>>      https://lore.kernel.org/linux-iommu/20200826025825.2322-1-
>> baolu.lu@...ux.intel.com/;
>>    - Add status check before disabling CFI. (Kevin)
>>
>> diff --git a/drivers/iommu/intel/irq_remapping.c
>> b/drivers/iommu/intel/irq_remapping.c
>> index 9564d23d094f..7552bb7e92c8 100644
>> --- a/drivers/iommu/intel/irq_remapping.c
>> +++ b/drivers/iommu/intel/irq_remapping.c
>> @@ -507,12 +507,19 @@ static void iommu_enable_irq_remapping(struct
>> intel_iommu *iommu)
>>
>>   	/* Enable interrupt-remapping */
>>   	iommu->gcmd |= DMA_GCMD_IRE;
>> -	iommu->gcmd &= ~DMA_GCMD_CFI;  /* Block compatibility-format
>> MSIs */
>>   	writel(iommu->gcmd, iommu->reg + DMAR_GCMD_REG);
>> -
>>   	IOMMU_WAIT_OP(iommu, DMAR_GSTS_REG,
>>   		      readl, (sts & DMA_GSTS_IRES), sts);
>>
>> +	/* Block compatibility-format MSIs */
>> +	sts = readl(iommu->reg + DMAR_GSTS_REG);
> 
> no need of this readl as the status is already three in IOMMU_WAIT_OP.

Yes. Really!

Best regards,
baolu

> 
>> +	if (sts & DMA_GSTS_CFIS) {
>> +		iommu->gcmd &= ~DMA_GCMD_CFI;
>> +		writel(iommu->gcmd, iommu->reg + DMAR_GCMD_REG);
>> +		IOMMU_WAIT_OP(iommu, DMAR_GSTS_REG,
>> +			      readl, !(sts & DMA_GSTS_CFIS), sts);
>> +	}
>> +
>>   	/*
>>   	 * With CFI clear in the Global Command register, we should be
>>   	 * protected from dangerous (i.e. compatibility) interrupts
>> --
>> 2.17.1
>