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

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Date:   Thu, 14 May 2020 18:05:16 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Michael Tsirkin <mst@...hat.com>,
        Julia Suvorova <jsuvorov@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.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 08:05:35PM +0200, Vitaly Kuznetsov wrote:
> The idea of the patchset was suggested by Michael S. Tsirkin.
> 
> PCIe config space can (depending on the configuration) be quite big but
> usually is sparsely populated. Guest may scan it by accessing individual
> device's page which, when device is missing, is supposed to have 'pci
> holes' semantics: reads return '0xff' and writes get discarded. Currently,
> userspace has to allocate real memory for these holes and fill them with
> '0xff'. Moreover, different VMs usually require different memory.
> 
> The idea behind the feature introduced by this patch is: let's have a
> single read-only page filled with '0xff' in KVM and map it to all such
> PCI holes in all VMs. This will free userspace of obligation to allocate
> real memory and also allow us to speed up access to these holes as we
> can aggressively map the whole slot upon first fault.
> 
> RFC. I've only tested the feature with the selftest (PATCH5) on Intel/AMD
> with and wiuthout EPT/NPT. I haven't tested memslot modifications yet.
> 
> Patches are against kvm/next.

Hi, Vitaly,

Could this be done in userspace with existing techniques?

E.g., shm_open() with a handle and fill one 0xff page, then remap it to
anywhere needed in QEMU?

Thanks,

-- 
Peter Xu