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

[Lu Baolu <baolu.lu@linux.intel.com>]

Hi Kevin,

On 8/26/20 1:29 PM, Tian, Kevin wrote:
>> From: Lu Baolu
>> Sent: Wednesday, August 26, 2020 10:58 AM
>>
>> 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@amacapital.net>
>> Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
>> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
>> ---
>>   drivers/iommu/intel/irq_remapping.c | 8 ++++++--
>>   1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/iommu/intel/irq_remapping.c
>> b/drivers/iommu/intel/irq_remapping.c
>> index 9564d23d094f..19d7e18876fe 100644
>> --- a/drivers/iommu/intel/irq_remapping.c
>> +++ b/drivers/iommu/intel/irq_remapping.c
>> @@ -507,12 +507,16 @@ 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 */
>> +	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);
>> +
> 
> Better do it only when CFI is actually enabled (by checking sts).

Yes. Make sense. Will send a new version with this changed.

Best regards,
baolu

> 
>>   	/*
>>   	 * With CFI clear in the Global Command register, we should be
>>   	 * protected from dangerous (i.e. compatibility) interrupts
>> --
>> 2.17.1
>>
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