Re: [PATCH 5/5] arm64: kdump: Don't defer the reservation of crash high memory

[Baoquan He]

On 06/23/22 at 03:07pm, Catalin Marinas wrote:
> On Wed, Jun 22, 2022 at 04:35:16PM +0800, Baoquan He wrote:
> > On 06/21/22 at 07:04pm, Catalin Marinas wrote:
> > > The problem with splitting is that you can end up with two entries in
> > > the TLB for the same VA->PA mapping (e.g. one for a 4KB page and another
> > > for a 2MB block). In the lucky case, the CPU will trigger a TLB conflict
> > > abort (but can be worse like loss of coherency).
> > 
> > Thanks for this explanation. Is this a drawback of arm64 design? X86
> > code do the same thing w/o issue, is there way to overcome this on
> > arm64 from hardware or software side?
> 
> It is a drawback of the arm64 implementations. Having multiple TLB
> entries for the same VA would need additional logic in hardware to
> detect, so the microarchitects have pushed back. In ARMv8.4, some
> balanced was reached with FEAT_BBM so that the only visible side-effect
> is a potential TLB conflict abort that could be resolved by software.

I see, thx.

> 
> > I ever got a arm64 server with huge memory, w or w/o crashkernel setting 
> > have different bootup time. And the more often TLB miss and flush will
> > cause performance cost. It is really a pity if we have very powerful
> > arm64 cpu and system capacity, but bottlenecked by this drawback.
> 
> Is it only the boot time affected or the runtime performance as well?

Sorry for late reply. What I observerd is the boot time serious latecy
with huge memory. Since the timestamp is not available at that time,
we can't tell the number. I didn't notice the runtime performance.