On Thu, Sep 26, 2013 at 08:55:24PM +0200, Borislav Petkov wrote:
On Thu, Sep 26, 2013 at 11:19:15AM -0300, Eduardo Habkost wrote:
Then we may have a problem: some CPU models already have "movbe" included (e.g. Haswell), and patch 6/6 will make "-cpu Haswell" get movbe enabled even if it is being emulated.
Huh? HSW has MOVBE so we won't #UD on it and MOVBE will get executed in hardware when executing the guest. IOW, we'll never get to the emulation path of piggybacking on the #UD.
So if we really want to avoid enabling emulated features by mistake, we may need a new CPU flag in addition to "enforce" to tell QEMU that it is OK to enable emulated features (maybe "-cpu ...,emulate"?).
EMULATED_CPUID are off by default and only if you request them specifically, they get enabled.
Please point me to the code that does this, because I don't see it on patch 6/6.
If you start with "-cpu Haswell", MOVBE will be already set in the host CPUID.
Or am I missing something?
In the Haswell example, it is unlikely but possible in theory: you would need a CPU that supported all features from Haswell except movbe. But what will happen if you are using "-cpu n270,enforce" on a SandyBridge host? Also, we don't know anything about future CPUs or future features that will end up on EMULATED_CPUID. The current code doesn't have anything to differentiate features that were already included in the CPU definition and ones explicitly enabled in the command-line (and I would like to keep it that way). And just because a feature was explicitly enabled in the command-line, that doesn't mean the user believe it is acceptable to get it running in emulated mode. That's why I propose a new "emulate" flag, to allow features to be enabled in emulated mode.
But my question still stands: suppose we had x2apic emulation implemented but for some reason it was painfully slow, we wouldn't want to enable it by mistake. In this case, it would end up on EMULATED_CPUID and not on SUPPORTED_CPUID, right?
IMHO we want to enable emulation only when explicitly requested... regardless of the emulation performance.
Well, x2apic is emulated by KVM, and it is on SUPPORTED_CPUID. Ditto for tsc-deadline. Or are you talking specifically about instruction emulation? -- Eduardo