Date:   Fri, 27 Sep 2019 15:52:58 +0800
From:   Kairui Song <kasong@...hat.com>
To:     Dave Young <dyoung@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Thomas Lendacky <Thomas.Lendacky@....com>,
        Baoquan He <bhe@...hat.com>, Lianbo Jiang <lijiang@...hat.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "kexec@...ts.infradead.org" <kexec@...ts.infradead.org>
Subject: Re: [PATCH v3 2/2] x86/kdump: Reserve extra memory when SME or SEV is active

On Fri, Sep 27, 2019 at 1:42 PM Dave Young <dyoung@...hat.com> wrote:
>
> On 09/25/19 at 06:36pm, Kairui Song wrote:
> > On Wed, Sep 11, 2019 at 1:56 PM Ingo Molnar <mingo@...nel.org> wrote:
> > > * Kairui Song <kasong@...hat.com> wrote:
> > >
> > > > Since commit c7753208a94c ("x86, swiotlb: Add memory encryption support"),
> > > > SWIOTLB will be enabled even if there is less than 4G of memory when SME
> > > > is active, to support DMA of devices that not support address with the
> > > > encrypt bit.
> > > >
> > > > And commit aba2d9a6385a ("iommu/amd: Do not disable SWIOTLB if SME is
> > > > active") make the kernel keep SWIOTLB enabled even if there is an IOMMU.
> > > >
> > > > Then commit d7b417fa08d1 ("x86/mm: Add DMA support for SEV memory
> > > > encryption") will always force SWIOTLB to be enabled when SEV is active
> > > > in all cases.
> > > >
> > > > Now, when either SME or SEV is active, SWIOTLB will be force enabled,
> > > > and this is also true for kdump kernel. As a result kdump kernel will
> > > > run out of already scarce pre-reserved memory easily.
> > > >
> > > > So when SME/SEV is active, reserve extra memory for SWIOTLB to ensure
> > > > kdump kernel have enough memory, except when "crashkernel=size[KMG],high"
> > > > is specified or any offset is used. As for the high reservation case, an
> > > > extra low memory region will always be reserved and that is enough for
> > > > SWIOTLB. Else if the offset format is used, user should be fully aware
> > > > of any possible kdump kernel memory requirement and have to organize the
> > > > memory usage carefully.
> > > >
> > > > Signed-off-by: Kairui Song <kasong@...hat.com>
> > > > ---
> > > >  arch/x86/kernel/setup.c | 20 +++++++++++++++++---
> > > >  1 file changed, 17 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> > > > index 71f20bb18cb0..ee6a2f1e2226 100644
> > > > --- a/arch/x86/kernel/setup.c
> > > > +++ b/arch/x86/kernel/setup.c
> > > > @@ -530,7 +530,7 @@ static int __init crashkernel_find_region(unsigned long long *crash_base,
> > > >                                         unsigned long long *crash_size,
> > > >                                         bool high)
> > > >  {
> > > > -     unsigned long long base, size;
> > > > +     unsigned long long base, size, mem_enc_req = 0;
> > > >
> > > >       base = *crash_base;
> > > >       size = *crash_size;
> > > > @@ -561,11 +561,25 @@ static int __init crashkernel_find_region(unsigned long long *crash_base,
> > > >       if (high)
> > > >               goto high_reserve;
> > > >
> > > > +     /*
> > > > +      * When SME/SEV is active and not using high reserve,
> > > > +      * it will always required an extra SWIOTLB region.
> > > > +      */
> > > > +     if (mem_encrypt_active())
> > > > +             mem_enc_req = ALIGN(swiotlb_size_or_default(), SZ_1M);
> > > > +
> > > >       base = memblock_find_in_range(CRASH_ALIGN,
> > > > -                                   CRASH_ADDR_LOW_MAX, size,
> > > > +                                   CRASH_ADDR_LOW_MAX,
> > > > +                                   size + mem_enc_req,
> > > >                                     CRASH_ALIGN);
> > >
> >
> > Hi Ingo,
> >
> > I re-read my previous reply, it's long and tedious, let me try to make
> > a more effective reply:
> >
> > > What sizes are we talking about here?
> >
> > The size here is how much memory will be reserved for kdump kernel, to
> > ensure kdump kernel and userspace can run without OOM.
> >
> > >
> > > - What is the possible size range of swiotlb_size_or_default()
> >
> > swiotlb_size_or_default() returns the swiotlb size, it's specified by
> > user using swiotlb=<size>, or default size (64MB)
> >
> > >
> > > - What is the size of CRASH_ADDR_LOW_MAX (the old limit)?
> >
> > It's 4G.
> >
> > >
> > > - Why do we replace one fixed limit with another fixed limit instead of
> > >   accurately sizing the area, with each required feature adding its own
> > >   requirement to the reservation size?
> >
> > It's quite hard to "accurately sizing the area".
> >
> > No way to tell the exact amount of memory kdump needs, we can only estimate.
> > Kdump kernel use different cmdline, drivers and components will have
> > special handling for kdump, and userspace is totally different.
>
> Agreed about your above, but specific this the problem in this patch
> There should be other ways.
>
> First thought about doing generic handling in swiotlb part, and do
> something like kdump_memory_reserve(size) Ingo suggested,  but according
> to you swiotlb init is late, so it can not increase the size, OTOH if
> reserve another region for kdump in swiotlb will cause other issues.
>
> So let's think about other improvement, for example to see if you can
> call kdump_memory_reserve(size) in AMD SME init path, for example in
> mem_encrypt_init(), is it before crashkernel reservation?

Yes, mem_encrypt_init is before the crashkernel reservation.
I think this is a good idea. Will make a V4 patch based on this.

>
> If doable it will be at least cleaner than the code in this patch.
>
> Thanks
> Dave

-- 
Best Regards,
Kairui Song

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