On Fri, Feb 23, 2018 at 10:37:49AM +0100, Paolo Bonzini wrote:
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
Explicit is fine.
But I would recommend you change the commit message to say so, and perhaps remove 'It is also slower' - as that is incorrect.
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.
<scratches his head> If it is detected to be Xen PV, then yes it will be a call to a function. But that won't help as Xen PV runs in ring 3, so it has a whole bunch of other issues.
If it detects it as KVM or Xen HVM guest it will patch it with the default - which is normal MSRs. Ditto for HyperV.
But <shrugs> no biggie - explicit is fine, just nagging on the commit message could use a bit of expansion.
Paolo