Re: [PATCH v5 00/13] KVM/X86: Introduce a new guest mapping interface

[Raslan, KarimAllah]

On Wed, 2019-01-30 at 18:14 +0100, Paolo Bonzini wrote:
> On 25/01/19 19:28, Raslan, KarimAllah wrote:
> > 
> > So the simple way to do it is:
> > 
> > 1- Pass 'mem=' in the kernel command-line to limit the amount of memory managedÂ
> > Â Âby the kernel.
> > 2- Map this physical memory you want to give to the guest with
> > Â Â Â mmap("/dev/mem", physical_address_offset, ..)
> > 3- Use the user-space virtual address as the "userspace_addr" fieldÂ
> > Â ÂinÂKVM_SET_USER_MEMORY_REGION ioctl.
> > 
> > You will also need this patch (hopefully I will repost next week as well):
> > https://patchwork.kernel.org/patch/9191755/
> 
> I took a look again at that patch and I guess I've changed my mind now
> that the kernel provides e820__mapped_any and e820__mapped_all.
> However, please do use e820__mapped_any instead of adding a new function
> e820_is_ram.

The problem withÂe820__mapped_* is that they are iterating over 'e820_table'
which is already truncated from the 'mem=' and 'memmap=' parameters:

"""
Â* - 'e820_table': this is the main E820 table that is massaged by the
Â*ÂÂÂlow level x86 platform code, or modified by boot parameters, before
Â*ÂÂÂpassed on to higher level MM layers.
"""

.. so I really still can not use it for this purpose. The structure that I want
to look at is actually 'e820_table_firmware' which is:

"""
Â* - 'e820_table_firmware': the original firmware version passed to us by the
Â*ÂÂÂbootloader - not modified by the kernel. It is composed of two parts:
Â*ÂÂÂthe first 128 E820 memory entries in boot_params.e820_table and the
remaining
Â*ÂÂÂ(if any) entries of the SETUP_E820_EXT nodes. We use this to:
Â*
Â*ÂÂÂÂÂÂÂ- inform the user about the firmware's notion of memory layout
Â*ÂÂÂÂÂÂÂÂÂvia /sys/firmware/memmap
Â*
Â*ÂÂÂÂÂÂÂ- the hibernation code uses it to generate a kernel-independent MD5
Â*ÂÂÂÂÂÂÂÂÂfingerprint of the physical memory layout of a system.
"""

The users ofÂe820__mapped_any expect these semantics, so even changing theÂ
implementation of these functions to use 'e820_table_firmware' to handle thisÂ
will not be an option!

One option here would be to add 'e820__mapped_raw_any' (or whateverÂ
other name) and make it identical to the current implementation ofÂ
e820__mapped_any at. Would that be slightly more acceptable? :)

> 
> Thanks,
> 
> Paolo
> 
> > 
> > I will make sure to expand on this in the cover letter in v6.
> 



Amazon Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrer: Christian Schlaeger, Ralf Herbrich
Ust-ID: DE 289 237 879
Eingetragen am Amtsgericht Charlottenburg HRB 149173 B