Re: [PATCH] iommu/mediatek: Use totalram_pages to setup enable_4GB

[Miles Chen <miles.chen@mediatek.com>]

On Thu, 2020-06-04 at 10:25 +0200, David Hildenbrand wrote:
> On 04.06.20 10:01, Miles Chen wrote:
> > To build this driver as a kernel module, we cannot use
> > the unexported symbol "max_pfn" to setup enable_4GB.
> > 
> > Use totalram_pages() instead to setup enable_4GB.
> > 
> > Suggested-by: Mike Rapoport <rppt@linux.ibm.com>
> > Signed-off-by: Miles Chen <miles.chen@mediatek.com>
> > Cc: David Hildenbrand <david@redhat.com>
> > Cc: Yong Wu <yong.wu@mediatek.com>
> > Cc: Chao Hao <chao.hao@mediatek.com>
> > ---
> >  drivers/iommu/mtk_iommu.c | 5 ++---
> >  1 file changed, 2 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/iommu/mtk_iommu.c b/drivers/iommu/mtk_iommu.c
> > index 5f4d6df59cf6..c2798a6e0e38 100644
> > --- a/drivers/iommu/mtk_iommu.c
> > +++ b/drivers/iommu/mtk_iommu.c
> > @@ -3,7 +3,6 @@
> >   * Copyright (c) 2015-2016 MediaTek Inc.
> >   * Author: Yong Wu <yong.wu@mediatek.com>
> >   */
> > -#include <linux/memblock.h>
> >  #include <linux/bug.h>
> >  #include <linux/clk.h>
> >  #include <linux/component.h>
> > @@ -626,8 +625,8 @@ static int mtk_iommu_probe(struct platform_device *pdev)
> >  		return -ENOMEM;
> >  	data->protect_base = ALIGN(virt_to_phys(protect), MTK_PROTECT_PA_ALIGN);
> >  
> > -	/* Whether the current dram is over 4GB */
> > -	data->enable_4GB = !!(max_pfn > (BIT_ULL(32) >> PAGE_SHIFT));
> > +	/* Whether the current dram is over 4GB, note: DRAM start at 1GB  */
> > +	data->enable_4GB = !!(totalram_pages() > ((SZ_2G + SZ_1G) >> PAGE_SHIFT));
> 
> A similar thing seems to be done by
> drivers/media/platform/mtk-vpu/mtk_vpu.c:
> 	vpu->enable_4GB = !!(totalram_pages() > (SZ_2G >> PAGE_SHIFT));
> 
> I do wonder if some weird memory hotplug setups might give you false
> negatives.
> 
> E.g., start a VM with 1GB and hotplug 1GB - it will be hotplugged on
> x86-64 above 4GB, turning max_pfn into 5GB. totalram_pages() should
> return something < 2GB.
> 
> Same can happen when you have a VM and use ballooning to fake-unplug
> memory, making totalram_pages() return something < 4GB, but leaving
> usable pfns >= 4GB

Yes. Yingjoe also told me that this patch is not correct.

Thanks for pointing this out. totalram_pages() does not work 
for some cases:

e.g., DRAM start @0x4000_0000 and DRAM size is 0x1_0000_0000 but we
reserve large amount of memory, which makes totalram_pages() < 3GB but
it is possible to allocate a pfn >= 4GB.

I will discuss this internally.

Miles
> .
> 
> but
> ... I don't know if I understood what "enable_4GB" needs/implies
> ... I don't know if this is applicable to VMs
>  at all (on real HW such
>     memory hotplug setups should not exist)
> ... I don't know how this code would react to memory hotplug, so if the
>     condition changes after the driver loaded and enable_4GB would
>     suddenly apply. Again, most probably not relevant on real HW, only
>     for VMs.
> 

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