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 managed
> > by the kernel.
> > 2- Map this physical memory you want to give to the guest with
> > mmap("/dev/mem", physical_address_offset, ..)
> > 3- Use the user-space virtual address as the "userspace_addr" field
> > in KVM_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 with e820__mapped_* is that they are iterating over 'e820_table'
which is already truncated from the 'mem=' and 'memmap=' parameters:
"""
* - 'e820_table': this is the main E820 table that is massaged by the
* low level x86 platform code, or modified by boot parameters, before
* passed 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:
"""
* - 'e820_table_firmware': the original firmware version passed to us by the
* bootloader - not modified by the kernel. It is composed of two parts:
* the first 128 E820 memory entries in boot_params.e820_table and the
remaining
* (if any) entries of the SETUP_E820_EXT nodes. We use this to:
*
* - inform the user about the firmware's notion of memory layout
* via /sys/firmware/memmap
*
* - the hibernation code uses it to generate a kernel-independent MD5
* fingerprint of the physical memory layout of a system.
"""
The users of e820__mapped_any expect these semantics, so even changing the
implementation of these functions to use 'e820_table_firmware' to handle this
will not be an option!
One option here would be to add 'e820__mapped_raw_any' (or whatever
other name) and make it identical to the current implementation of
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.
>
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