CLFLUSH in virtualization environment

Question

I have read the document about the CLFLUSH instruction of Intel x86 machine. I understand that CLFLUSH m8 means flush the cache line containing linear address m8 if I run the instruction inside a Linux module on the bare metal.

However, what if I want to run the instruction inside a VM in virtualization environment? What the parameter of CLFLUSH is? To be specific, suppose I want to flush a cache line that hold the content for virtual address va of a program in a VM. The virtual address va is mapped to physical address pa in the VM, and pa will be mapped to the machine address ma in the VMM/hypervisor. Which address should I use for the CLFLUSH instruction, va, pa or ma?

Thank you very much!

Answer 1

clflush is not a privileged instruction. User processes can run it, just like they can run load, store, prefetch, and movnt (store with cache eviction) instructions which also affect the cache.

I expect it virtualizes just fine, without any hypervisor support, since it uses addresses the same way other memory references do.

Since hardware virtualization allows the CPU to do a full translation from guest virtual addresses to hardware physical addresses, the desired behaviour (cache line synced to physical memory) should happen without hypervisor intervention.