lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
Open Source and information security mailing list archives
| ||
|
Date: Thu, 4 Apr 2013 15:19:34 +0300 From: Gleb Natapov <gleb@...hat.com> To: Alexander Graf <agraf@...e.de> Cc: "Michael S. Tsirkin" <mst@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org, Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>, Takuya Yoshikawa <yoshikawa_takuya_b1@....ntt.co.jp>, Alex Williamson <alex.williamson@...hat.com>, Will Deacon <will.deacon@....com>, Christoffer Dall <c.dall@...tualopensystems.com>, Sasha Levin <sasha.levin@...cle.com>, Andrew Morton <akpm@...ux-foundation.org>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, virtualization@...ts.linux-foundation.org Subject: Re: [PATCH RFC] kvm: add PV MMIO EVENTFD On Thu, Apr 04, 2013 at 02:09:53PM +0200, Alexander Graf wrote: > > On 04.04.2013, at 13:04, Michael S. Tsirkin wrote: > > > On Thu, Apr 04, 2013 at 01:57:34PM +0200, Alexander Graf wrote: > >> > >> On 04.04.2013, at 12:50, Michael S. Tsirkin wrote: > >> > >>> With KVM, MMIO is much slower than PIO, due to the need to > >>> do page walk and emulation. But with EPT, it does not have to be: we > >>> know the address from the VMCS so if the address is unique, we can look > >>> up the eventfd directly, bypassing emulation. > >>> > >>> Add an interface for userspace to specify this per-address, we can > >>> use this e.g. for virtio. > >>> > >>> The implementation adds a separate bus internally. This serves two > >>> purposes: > >>> - minimize overhead for old userspace that does not use PV MMIO > >>> - minimize disruption in other code (since we don't know the length, > >>> devices on the MMIO bus only get a valid address in write, this > >>> way we don't need to touch all devices to teach them handle > >>> an dinvalid length) > >>> > >>> At the moment, this optimization is only supported for EPT on x86 and > >>> silently ignored for NPT and MMU, so everything works correctly but > >>> slowly. > >>> > >>> TODO: NPT, MMU and non x86 architectures. > >>> > >>> The idea was suggested by Peter Anvin. Lots of thanks to Gleb for > >>> pre-review and suggestions. > >>> > >>> Signed-off-by: Michael S. Tsirkin <mst@...hat.com> > >> > >> This still uses page fault intercepts which are orders of magnitudes > >> slower than hypercalls. > > > > Not really. Here's a test: > > compare vmcall to portio: > > > > vmcall 1519 > > ... > > outl_to_kernel 1745 > > > > compare portio to mmio: > > > > mmio-wildcard-eventfd:pci-mem 3529 > > mmio-pv-eventfd:pci-mem 1878 > > portio-wildcard-eventfd:pci-io 1846 > > > > So not orders of magnitude. > > https://dl.dropbox.com/u/8976842/KVM%20Forum%202012/MMIO%20Tuning.pdf > > Check out page 41. Higher is better (number is number of loop cycles in a second). My test system was an AMD Istanbul based box. > Have you bypassed instruction emulation in your testing? > > > >> Why don't you just create a PV MMIO hypercall > >> that the guest can use to invoke MMIO accesses towards the host based > >> on physical addresses with explicit length encodings? > >> That way you simplify and speed up all code paths, exceeding the speed > >> of PIO exits even. It should also be quite easily portable, as all > >> other platforms have hypercalls available as well. > >> > >> > >> Alex > > > > I sent such a patch, but maintainers seem reluctant to add hypercalls. > > Gleb, could you comment please? > > > > A fast way to do MMIO is probably useful in any case ... > > Yes, but at least according to my numbers optimizing anything that is not hcalls is a waste of time. > > > Alex -- Gleb. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Powered by blists - more mailing lists