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From:   Pingfan Liu <kernelfans@gmail.com>
Date:   Wed, 20 Feb 2019 15:38:43 +0800
Message-ID: <CAFgQCTu7pS=f2NgNvUgWaSt0hQ=KOZJXjBxktLi4PBkuXwvDFA@mail.gmail.com>
Subject: Re: [PATCHv7] x86/kdump: bugfix, make the behavior of crashkernel=X
 consistent with kaslr
To:     Dave Young <dyoung@redhat.com>
Cc:     Borislav Petkov <bp@alien8.de>, Baoquan He <bhe@redhat.com>,
        Jerry Hoemann <jerry.hoemann@hpe.com>, x86@kernel.org,
        Randy Dunlap <rdunlap@infradead.org>,
        kexec@lists.infradead.org, LKML <linux-kernel@vger.kernel.org>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Yinghai Lu <yinghai@kernel.org>, vgoyal@redhat.com,
        iommu@lists.linux-foundation.org, konrad.wilk@oracle.com
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On Mon, Feb 18, 2019 at 9:48 AM Dave Young <dyoung@redhat.com> wrote:
>
> On 02/15/19 at 11:24am, Borislav Petkov wrote:
> > On Tue, Feb 12, 2019 at 04:48:16AM +0800, Dave Young wrote:
> > > Even we make it automatic in kernel, but we have to have some default
> > > value for swiotlb in case crashkernel can not find a free region under 4G.
> > > So this default value can not work for every use cases, people need
> > > manually use crashkernel=,low and crashkernel=,high in case
> > > crashkernel=X does not work.
> >
> > Why would the user need to find swiotlb range? The kernel has all the
> > information it requires at its finger tips in order to decide properly.
> >
> > The user wants a crashkernel range, the kernel tries the low range =>
> > no workie, then it tries the next range => workie but needs to allocate
> > swiotlb range so that DMA can happen too. Doh, then the kernel does
> > allocate that too.
>
> It is ideal if kernel can do it automatically, but I'm not sure if
> kernel can predict the swiotlb reserved size automatically.
>
Agreed, I think it is hard to decide the reserved size automatically.
We do not know the requirement for memory of ZONE_DMA32 at boot time.
The requirement depends on how many DMA32 devices, and the dynamic
payload of them.

> Let's add more people to seek for comments.
>
> >
> > Why would the user need to do anything here?!
> >
> > --
> > Regards/Gruss,
> >     Boris.
> >
> > Good mailing practices for 400: avoid top-posting and trim the reply.