Re: [Qemu-devel] [RFC PATCH 1/4] pc-dimm: add 'reserved-size' to reserve address range after the ending address

[Haozhong Zhang]

On 04/07/17 14:46 +0100, Stefan Hajnoczi wrote:
> On Thu, Apr 06, 2017 at 06:46:49PM +0800, Haozhong Zhang wrote:
> > On 04/06/17 11:24 +0100, Stefan Hajnoczi wrote:
> > > On Fri, Mar 31, 2017 at 04:41:44PM +0800, Haozhong Zhang wrote:
> > > > If option 'reserved-size=RSVD' is present, QEMU will reserve an
> > > > address range of size 'RSVD' after the ending address of pc-dimm
> > > > device.
> > > > 
> > > > For the following example,
> > > >     -object memory-backend-file,id=mem0,size=4G,...
> > > >     -device nvdimm,id=dimm0,memdev=mem0,reserved-size=4K,...
> > > 
> > > "reserved-size" is not a clear name.  I suggest calling the property
> > > "num-flush-hints" (default 0).  QEMU can calculate the actual size in
> > > bytes.
> > > 
> > >   -device nvdimm,num-flush-hints=1
> > > 
> > > QEMU will use one flush hint and reserve enough space (e.g. 1 page) for
> > > the MMIO region.
> > >
> > 
> > Each flush hint can be as small as one cache line size which is also
> > the size used in this patch series.
> > 
> > We need to calculate the size of all flush hints in pc_dimm_memory_plug(),
> > because when building the flush hint address structure we need to know
> > the address of flush hints.
> > 
> > IIUC, pc_dimm_memory_plug() is not specific to x86, so it's better
> > take a general way to get the vcpu cache line size in pc_dimm_memory_plug(),
> > which seemingly lacks in QEMU (though I believe it should be doable).
> > 
> > To make things simple, I leave the size decision to users, and check
> > whether it's large enough when building the flush hint address
> > structures in patch 4.
> 
> Do you see my concern that "reserved-size" is not a good property?
> 
>  * How does the user choose a sensible value?

A user has to calculate the size by himself by multiplying the number
of flush hint address (currently it's hardcoded to 1) with the cache
line size (e.g. 64 bytes on x86) and properly align it, or simply uses
4KB or even a larger value.

In case the reserved-size is too small, QEMU will complain and abort,
so the user still has chance to know what goes wrong.

> 
>  * Why is it called "reserved-size" instead of "flush-hints-size"?
>

This property is made as a property of pc-dimm rather than nvdimm,
which could be used for other purposes, e.g. adjust the alignment the
base address of hotplugged pc-dimm (I remember old linux kernels on
x86-64 requires hotplugged memory chunk be 128MB aligned). Of course,
it can be limited to be a nvdimm-only property, and renamed to
'flush-hints-size' in that case.

Thanks,
Haozhong