On Fri, Feb 14, 2020 at 08:59:39AM -0800, Dave Hansen wrote: > On 2/13/20 11:23 PM, Joerg Roedel wrote: > > Yes, investigating this is on the list for future optimizations (besides > > caching CPUID results). My idea is to use alternatives patching for > > this. But the exception handling is needed anyway because #VC > > exceptions happen very early already, basically the first thing after > > setting up a stack is calling verify_cpu(), which uses CPUID. > > Ahh, bummer. How does a guest know that it's running under SEV-ES? > What's the enumeration mechanism if CPUID doesn't "work"? There are two ways a guest can find out: 1) Read the SEV_STATUS_MSR and check for the SEV-ES bit 2) If a #VC exception is raised it also knows it runs as an SEV-ES guest This patch-set implements both ways at the appropriate stages of the boot process. Very early it just installs a #VC handler without checking whether it is running under SEV-ES and handles the exceptions when they are raised. Later in the boot process it also reads the SEV_STATUS_MSR and sets a cpu_feature flag to do alternative patching based on its value. > > The other reason is that things like MMIO and IOIO instructions can't be > > easily patched by alternatives. Those would work with the runtime > > checking you showed above, though. > > Is there a reason we can't make a rule that you *must* do MMIO through > an accessor function so we *can* patch them? I know random drivers > might break the rule, but are SEV-ES guests going to be running random > drivers? I would think that they mostly if not all want to use > virtio. Yeah, there are already defined accessor functions for MMIO, like read/write[bwlq] and memcpy_toio/memcpy_fromio. It is probably worth testing what performance overhead is involved in overloading these to call directly into the paravirt path when SEV-ES is enabled. With alternatives patching it would still add a couple of NOPS for the non-SEV-ES case. But all that does not remove the need for the #VC exception handler, as #VC exceptions can also be triggered by user-space, and the instruction emulation for MMIO will be needed to allow MMIO in user-space (the patch-set currently does not allow that, but it could be needed in the future). Regards, Joerg _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
