On 2/11/20 5:52 AM, Joerg Roedel wrote:> Implement a handler for #VC exceptions caused by RDMSR/WRMSR > instructions.As a general comment on all of these event handlers: Why do we bother having the hypercalls in the interrupt handler as opposed to just calling them directly. What you have is: wrmsr() -> #VC exception hcall() But we could make our rd/wrmsr() wrappers just do: if (running_on_sev_es()) hcall(HCALL_MSR_WHATEVER...) else wrmsr() and then we don't have any of the nastiness of exception handling.
On Thu, Feb 13, 2020 at 07:45:00AM -0800, Dave Hansen wrote:> On 2/11/20 5:52 AM, Joerg Roedel wrote: > > Implement a handler for #VC exceptions caused by RDMSR/WRMSR > > instructions. > > As a general comment on all of these event handlers: Why do we bother > having the hypercalls in the interrupt handler as opposed to just > calling them directly. What you have is: > > wrmsr() > -> #VC exception > hcall() > > But we could make our rd/wrmsr() wrappers just do: > > if (running_on_sev_es()) > hcall(HCALL_MSR_WHATEVER...) > else > wrmsr() > > and then we don't have any of the nastiness of exception handling.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. 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. Regards, Joerg
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"?> 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.