On 2011-08-29 13:11, Avi Kivity wrote: > On 08/29/2011 02:03 PM, Jan Kiszka wrote: >>> >>> Just reading the first byte requires a guest page table walk. This is >>> probably the highest cost in emulation (which also requires a walk for >>> the data access). >> >> And what about caching the result of the first walk? Usually, a "sane >> guest" won't have many code pages that issue the EIO. >> > > There's no way to know when to invalidate the cache. Set the affected code page read-only? > > We could go a bit further, and cache the the whole thing. On the first > exit, do the entire emulation, and remember %rip. On the second exit, > if %rip matches, skip directly to kvm_lapic_eoi(). > > But I don't think it's worth it. This also has failure modes, and > really, no guest will ever write to EOI with stosl. ...or add/sub/and/or etc. Well, we've done other crazy things in the past just to keep even the unlikely case correct. I was just wondering if that policy changed. However, I just realized that user space is able to avoid this inaccuracy for potentially insane guests by not using in-kernel irqchips. So we have at least a knob. Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
