[kvm-devel] kvm_regs vs kvm_sregs?
What is the distinction between kvm_regs and kvm_sregs? As far as I can see, kvm_regs is only used when emulating IO, emulating MMIO, and emulating CPUID, where guest GPRs are directly modified. kvm_sregs is only used for full CPU state save (for later restore). When the kernel had to use copy_to_user() to transfer state to userland, I can see that this split resulted in less memory copying. However, now that userland can directly map register state without a copy, why not combine the two structures? I guess it takes longer to copy state out of the VMCS into kvm_sregs, so why bother if userspace isn't going to use it? -Hollis - Take Surveys. Earn Cash. Influence the Future of IT Join SourceForge.net's Techsay panel and you'll get the chance to share your opinions on IT & business topics through brief surveys-and earn cash http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV ___ kvm-devel mailing list kvm-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kvm-devel
Re: [kvm-devel] kvm_regs vs kvm_sregs?
Hollis Blanchard wrote: > What is the distinction between kvm_regs and kvm_sregs? As far as I can > see, kvm_regs is only used when emulating IO, emulating MMIO, and > emulating CPUID, where guest GPRs are directly modified. kvm_sregs is > only used for full CPU state save (for later restore). > After the userspace interface changes, neither of them are used except for migration, savevm/loadvm, and debugging. Theoretically they could be unified, practically SET_SREGS is a dangerous operation on Intel cpus due to kvm's very imperfect real mode support. > When the kernel had to use copy_to_user() to transfer state to userland, > I can see that this split resulted in less memory copying. However, now > that userland can directly map register state without a copy, why not > combine the two structures? > We don't map the register state as that would require us to sync it on every exit. > I guess it takes longer to copy state out of the VMCS into kvm_sregs, so > why bother if userspace isn't going to use it? > When it's needed, it's really needed. Or did I misunderstand the question? -- Do not meddle in the internals of kernels, for they are subtle and quick to panic. - Take Surveys. Earn Cash. Influence the Future of IT Join SourceForge.net's Techsay panel and you'll get the chance to share your opinions on IT & business topics through brief surveys-and earn cash http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV ___ kvm-devel mailing list kvm-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kvm-devel
Re: [kvm-devel] kvm_regs vs kvm_sregs?
On Sun, 2007-03-18 at 07:13 +0200, Avi Kivity wrote: > Hollis Blanchard wrote: > > What is the distinction between kvm_regs and kvm_sregs? As far as I can > > see, kvm_regs is only used when emulating IO, emulating MMIO, and > > emulating CPUID, where guest GPRs are directly modified. kvm_sregs is > > only used for full CPU state save (for later restore). > > > > After the userspace interface changes, neither of them are used except > for migration, savevm/loadvm, and debugging. Theoretically they could be > unified, practically SET_SREGS is a dangerous operation on Intel cpus > due to kvm's very imperfect real mode support. > > > When the kernel had to use copy_to_user() to transfer state to userland, > > I can see that this split resulted in less memory copying. However, now > > that userland can directly map register state without a copy, why not > > combine the two structures? > > > > We don't map the register state as that would require us to sync it on > every exit. > > > I guess it takes longer to copy state out of the VMCS into kvm_sregs, so > > why bother if userspace isn't going to use it? > > > > When it's needed, it's really needed. Or did I misunderstand the question? I guess I misunderstood. I thought the mmap was for userspace to access kvm_regs, which previously was needed when emulating IO. Apparently this is no longer the case: instead the data is copied somewhere into mmap space and then complete_pio() is responsible for loading that raw data into the guest register state. Seems a bit more convoluted, but I guess you've managed to divorce user and kernel data structures, if that was the goal. -Hollis - Take Surveys. Earn Cash. Influence the Future of IT Join SourceForge.net's Techsay panel and you'll get the chance to share your opinions on IT & business topics through brief surveys-and earn cash http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV ___ kvm-devel mailing list kvm-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kvm-devel
