linux-kernel - Re: [PATCH v8 10/13] iommu/amd: Allocate memory below 4G for dev table if translation pre-enabled

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Message-ID: <20170731102925.GA9999@x1>
Date:   Mon, 31 Jul 2017 18:29:25 +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 v8 10/13] iommu/amd: Allocate memory below 4G for dev
 table if translation pre-enabled

On 07/31/17 at 12:21pm, Joerg Roedel wrote:
> Hi Baoquan,
> 
> On Mon, Jul 31, 2017 at 06:15:30PM +0800, Baoquan He wrote:
> > I plan to add GFP_DMA32 when allocate amd_iommu_dev_table in
> > early_amd_iommu_init() as below. Then in kdump kernel we don't need to
> > worry if the old amd_iommu_dev_table could be above 4G, right? And might
> > not need to check if it's above 4G, right?
> > 
> > diff --git a/drivers/iommu/amd_iommu_init.c b/drivers/iommu/amd_iommu_init.c
> > index 781a138..85d6445 100644
> > --- a/drivers/iommu/amd_iommu_init.c
> > +++ b/drivers/iommu/amd_iommu_init.c
> > @@ -2436,7 +2436,8 @@ static int __init early_amd_iommu_init(void)
> >  
> >  	/* Device table - directly used by all IOMMUs */
> >  	ret = -ENOMEM;
> > -	amd_iommu_dev_table = (void *)__get_free_pages(GFP_KERNEL | __GFP_ZERO,
> > +	amd_iommu_dev_table = (void *)__get_free_pages(
> > +				      GFP_KERNEL | __GFP_ZERO | GFP_DMA32,
> >  				      get_order(dev_table_size));
> >  	if (amd_iommu_dev_table == NULL)
> >  		goto out;
> 
> Yeah, adding GFP_DMA32 is right. But you still need to check it in the
> kdump path. Not checking it would mean you trust the old kernel, but
> since it paniced there is no reason to put any trust in what happened
> before.

You are right, it could be touched accidentally. It must be checked.
Thanks a lot for your answer!