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

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Message-ID: <1548440932.17444.29.camel@amazon.de>
Date:   Fri, 25 Jan 2019 18:28:52 +0000
From:   "Raslan, KarimAllah" <karahmed@...zon.de>
To:     "konrad.wilk@...cle.com" <konrad.wilk@...cle.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>
Subject: Re: [PATCH v5 00/13] KVM/X86: Introduce a new guest mapping interface

On Wed, 2019-01-23 at 13:16 -0500, Konrad Rzeszutek Wilk wrote:
> On Wed, Jan 09, 2019 at 10:42:00AM +0100, KarimAllah Ahmed wrote:
> > 
> > Guest memory can either be directly managed by the kernel (i.e. have a "struct
> > page") or they can simply live outside kernel control (i.e. do not have a
> > "struct page"). KVM mostly support these two modes, except in a few places
> > where the code seems to assume that guest memory must have a "struct page".
> > 
> > This patchset introduces a new mapping interface to map guest memory into host
> > kernel memory which also supports PFN-based memory (i.e. memory without 'struct
> > page'). It also converts all offending code to this interface or simply
> > read/write directly from guest memory. Patch 2 is additionally fixing an
> > incorrect page release and marking the page as dirty (i.e. as a side-effect of
> > using the helper function to write).
> > 
> > As far as I can see all offending code is now fixed except the APIC-access page
> > which I will handle in a seperate series along with dropping
> > kvm_vcpu_gfn_to_page and kvm_vcpu_gpa_to_page from the internal KVM API.
> > 
> > The current implementation of the new API uses memremap to map memory that does
> > not have a "struct page". This proves to be very slow for high frequency
> > mappings. Since this does not affect the normal use-case where a "struct page"
> > is available, the performance of this API will be handled by a seperate patch
> > series.
> 
> Where could one find this patchset?

Let me clean it and send it out as well :)

> 
> Also is there an simple test-case (or a writeup) you have for testing
> this code? Specifically I am thinking about the use-case of "memory
> without the 'struct page'"

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 will make sure to expand on this in the cover letter in v6.

> 
> And thank you for posting this patchset. It was a pleasure reviewing the
> code!



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