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

[Lu Baolu]

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@xxxxxxxxxxxxxx>
> > Cc: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
> > Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> > ---
> >   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
> >
> > _______________________________________________
> > iommu mailing list
> > iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx
> > https://lists.linuxfoundation.org/mailman/listinfo/iommu