Hi, In the recent KVM forum, Marcelo Tosatti presented some KVM performance improvements. One of them (page 17 of the presentation) was about the MSRs used by SYSCALL: MSR_STAR, MSR_LSTAR, MSR_CSTAR (and also MSR_SYSCALL_MASK). He said that "Guests have direct access to these MSRs", and this is why KVM needs to restore their original host value when returning to user space (they aren't used in the kernel, so there's no reason to restore them earlier). I was suprised, then, to discover that KVM doesn't add these MSRs to the MSR bitmap, so when the guest changes these MSR values, or even reads them, we cause an exit - both on read and on write of these MSRs. I was wondering why these exits are needed. I can maybe guess why an exit is needed on write - just to save the guest value so we don't need to read it when going back to user space. Is this a good optimization because we assume that the guest very rarely changes these MSRs? Or is there another explanation as to why these exits are needed? But I can't even guess why an exit is needed on read... If you're curious why I noticed these exits - I was running a nested KVM (L0's guest L1 is KVM, that itself has a guest L2). Whenever L2 does something that L1 needs to handle in user space (e.g., PIO), L1 does all these MSR reads and writes, and we get exits for each of them - many of those exits for each L2 PIO :( Thanks, Nadav Har'El. -- Nadav Har'El | Wednesday, Feb 16 2011, 12 Adar I 5771 nyh@xxxxxxxxxxxxxxxxxxx |----------------------------------------- Phone +972-523-790466, ICQ 13349191 |Support bacteria - they're the only http://nadav.harel.org.il |culture some people have! -- 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
