From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
Michael Tsirkin <mst@redhat.com>,
Julia Suvorova <jsuvorov@redhat.com>,
Paolo Bonzini <pbonzini@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>,
x86@kernel.org
Subject: Re: [PATCH RFC 2/5] KVM: x86: introduce KVM_MEM_ALLONES memory
Date: Fri, 15 May 2020 10:24:45 +0200 [thread overview]
Message-ID: <87imgxwqpe.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <20200514191823.GA15847@linux.intel.com>
Sean Christopherson <sean.j.christopherson@intel.com> writes:
> On Thu, May 14, 2020 at 08:05:37PM +0200, Vitaly Kuznetsov wrote:
>> 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. Later, this will also allow us to speed up access to these
>> holes as we can aggressively map the whole slot upon first fault.
>>
>> Suggested-by: Michael S. Tsirkin <mst@redhat.com>
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> ---
>> Documentation/virt/kvm/api.rst | 22 ++++++---
>> arch/x86/include/uapi/asm/kvm.h | 1 +
>> arch/x86/kvm/x86.c | 9 ++--
>> include/linux/kvm_host.h | 15 ++++++-
>> include/uapi/linux/kvm.h | 2 +
>> virt/kvm/kvm_main.c | 79 +++++++++++++++++++++++++++++++--
>> 6 files changed, 113 insertions(+), 15 deletions(-)
>>
>> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
>> index d871dacb984e..2b87d588a7e0 100644
>> --- a/Documentation/virt/kvm/api.rst
>> +++ b/Documentation/virt/kvm/api.rst
>> @@ -1236,7 +1236,8 @@ yet and must be cleared on entry.
>>
>> /* for kvm_memory_region::flags */
>> #define KVM_MEM_LOG_DIRTY_PAGES (1UL << 0)
>> - #define KVM_MEM_READONLY (1UL << 1)
>> + #define KVM_MEM_READONLY (1UL << 1)
>> + #define KVM_MEM_ALLONES (1UL << 2)
>
> Why not call this KVM_MEM_PCI_HOLE or something else that better conveys
> that this is memslot is intended to emulate PCI master abort semantics?
>
Becuase there's always hope this can be usefult for something else but
PCI? :-) Actually, I was thinking about generalizing this a little bit
to something like KVM_MEM_CONSTANT with a way to set the pattern but I'm
failing to see any need for anything but all-ones or all-zeroes. Maybe
other-than-x86 architectures have some needs?
I'm definitely fine with renaming this to KVM_MEM_PCI_HOLE.
--
Vitaly
next prev parent reply other threads:[~2020-05-15 8:24 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-14 18:05 [PATCH RFC 0/5] KVM: x86: KVM_MEM_ALLONES memory Vitaly Kuznetsov
2020-05-14 18:05 ` [PATCH RFC 1/5] KVM: rename labels in kvm_init() Vitaly Kuznetsov
2020-05-14 18:05 ` [PATCH RFC 2/5] KVM: x86: introduce KVM_MEM_ALLONES memory Vitaly Kuznetsov
2020-05-14 19:18 ` Sean Christopherson
2020-05-15 8:24 ` Vitaly Kuznetsov [this message]
2020-05-14 18:05 ` [PATCH RFC 3/5] KVM: x86: move kvm_vcpu_gfn_to_memslot() out of try_async_pf() Vitaly Kuznetsov
2020-05-14 18:05 ` [PATCH RFC 4/5] KVM: x86: aggressively map PTEs in KVM_MEM_ALLONES slots Vitaly Kuznetsov
2020-05-14 19:46 ` Sean Christopherson
2020-05-15 8:36 ` Vitaly Kuznetsov
2020-05-15 13:58 ` Sean Christopherson
2020-05-14 18:05 ` [PATCH RFC 5/5] KVM: selftests: add KVM_MEM_ALLONES test Vitaly Kuznetsov
2020-05-14 22:05 ` [PATCH RFC 0/5] KVM: x86: KVM_MEM_ALLONES memory Peter Xu
2020-05-14 22:56 ` Sean Christopherson
2020-05-14 23:22 ` Peter Xu
2020-05-14 23:32 ` Sean Christopherson
2020-05-15 8:42 ` Vitaly Kuznetsov
2020-05-15 1:03 ` Andy Lutomirski
2020-05-15 11:15 ` Peter Xu
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