linux-kernel - Re: [PATCH RFC 0/5] KVM: x86: KVM_MEM

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Date:   Thu, 14 May 2020 16:32:08 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Michael Tsirkin <mst@...hat.com>,
        Julia Suvorova <jsuvorov@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>, x86@...nel.org
Subject: Re: [PATCH RFC 0/5] KVM: x86: KVM_MEM_ALLONES memory

On Thu, May 14, 2020 at 07:22:50PM -0400, Peter Xu wrote:
> On Thu, May 14, 2020 at 03:56:24PM -0700, Sean Christopherson wrote:
> > On Thu, May 14, 2020 at 06:05:16PM -0400, Peter Xu wrote:
> > > E.g., shm_open() with a handle and fill one 0xff page, then remap it to
> > > anywhere needed in QEMU?
> > 
> > Mapping that 4k page over and over is going to get expensive, e.g. each
> > duplicate will need a VMA and a memslot, plus any PTE overhead.  If the
> > total sum of the holes is >2mb it'll even overflow the mumber of allowed
> > memslots.
> 
> What's the PTE overhead you mentioned?  We need to fill PTEs one by one on
> fault even if the page is allocated in the kernel, am I right?

It won't require host PTEs for every page if it's a kernel page.  I doubt
PTEs are a significant overhead, especially compared to memslots, but it's
still worth considering.

My thought was to skimp on both host PTEs _and_ KVM SPTEs by always sending
the PCI hole accesses down the slow MMIO path[*].

[*] https://lkml.kernel.org/r/20200514194624.GB15847@linux.intel.com

> 4K is only an example - we can also use more pages as the template.  However I
> guess the kvm memslot count could be a limit..  Could I ask what's the normal
> size of this 0xff region, and its distribution?
> 
> Thanks,
> 
> -- 
> Peter Xu
>