linux-kernel - Re: [PATCH v9 10/13] iommu/amd: Allocate memory below 4G for dev table if translation pre-enabled

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Date:   Fri, 4 Aug 2017 21:01:12 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Joerg Roedel <jroedel@...e.de>
Cc:     iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 10/13] iommu/amd: Allocate memory below 4G for dev
 table if translation pre-enabled

On 08/04/17 at 02:25pm, Joerg Roedel wrote:
> On Tue, Aug 01, 2017 at 07:37:26PM +0800, Baoquan He wrote:
> > AMD pointed out it's unsafe to update the device-table while iommu
> > is enabled. It turns out that device-table pointer update is split
> > up into two 32bit writes in the IOMMU hardware. So updating it while
> > the IOMMU is enabled could have some nasty side effects.
> > 
> > The safe way to work around this is to always allocate the device-table
> > below 4G, including the old device-table in normal kernel and the
> > device-table used for copying the content of the old device-table in kdump
> > kernel. Meanwhile we need check if the address of old device-table is
> > above 4G because it might has been touched accidentally in corrupted
> > 1st kernel.
> > 
> > Signed-off-by: Baoquan He <bhe@...hat.com>
> > ---
> >  drivers/iommu/amd_iommu_init.c | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/iommu/amd_iommu_init.c b/drivers/iommu/amd_iommu_init.c
> > index 6a77b99d08e4..8c6431ac5698 100644
> > --- a/drivers/iommu/amd_iommu_init.c
> > +++ b/drivers/iommu/amd_iommu_init.c
> > @@ -882,11 +882,15 @@ static int copy_device_table(void)
> >  			continue;
> >  
> >  		old_devtb_phys = entry & PAGE_MASK;
> > +		if (old_devtb_phys > 0x100000000ULL) {
> 
> Needs to be '>='.

Will change.

> 
> > +			pr_err("The address of old device table is above 4G, not trustworthy!/n");
> > +			return -1;
> > +		}
> 
> Okay, forget my previous comment about it, the check is added here
>