On 22/02/2018 18:07, Konrad Rzeszutek Wilk wrote:
Having a paravirt indirect call in the IBRS restore path is not a good idea, since we are trying to protect from speculative execution of bogus indirect branch targets. It is also slower, so use native_wrmsrl on the vmentry path too.
But it gets replaced during patching. As in once the machine boots the assembler changes from:
callq *0xfffflbah
to wrmsr
? I don't think you need this patch.
Why not be explicit? According to the spec, PRED_CMD and SPEC_CTRL should be passed down to the guest without interception so it's safe to do this. On the other hand, especially with nested virtualization, I don't think you can absolutely guarantee that the paravirt call will be patched to rdmsr/wrmsr.
Paolo