On Tue, Jun 23, 2020 at 8:23 AM Andrew Cooper [off-list ref] wrote:

On 23/06/2020 14:03, Peter Zijlstra wrote:

quoted

On Tue, Jun 23, 2020 at 02:12:37PM +0200, Joerg Roedel wrote:

quoted

On Tue, Jun 23, 2020 at 01:50:14PM +0200, Peter Zijlstra wrote:

quoted

If SNP is the sole reason #VC needs to be IST, then I'd strongly urge
you to only make it IST if/when you try and make SNP happen, not before.

It is not the only reason, when ES guests gain debug register support
then #VC also needs to be IST, because #DB can be promoted into #VC
then, and as #DB is IST for a reason, #VC needs to be too.

Didn't I read somewhere that that is only so for Rome/Naples but not for
the later chips (Milan) which have #DB pass-through?

I don't know about hardware timelines, but some future part can now opt
in to having debug registers as part of the encrypted state, and swapped
by VMExit, which would make debug facilities generally usable, and
supposedly safe to the #DB infinite loop issues, at which point the
hypervisor need not intercept #DB for safety reasons.

Its worth nothing that on current parts, the hypervisor can set up debug
facilities on behalf of the guest (or behind its back) as the DR state
is unencrypted, but that attempting to intercept #DB will redirect to
#VC inside the guest and cause fun. (Also spare a thought for 32bit
kernels which have to cope with userspace singlestepping the SYSENTER
path with every #DB turning into #VC.)

What do you mean 32-bit?  64-bit kernels have exactly the same
problem.  At least the stack is okay, though.


Anyway, since I'm way behind on this thread, here are some thoughts:

First, I plan to implement actual precise recursion detection for the
IST stacks.  We'll be able to reliably panic when unallowed recursion
happens.

Second, I don't object *that* strongly to switching to a second #VC
stack if an NMI or MCE happens, but we really need to make sure we
cover *all* the bases.  And #VC is distressingly close to "happens at
all kinds of unfortunate times and the guest doesn't actually have
much ability to predice it" right now.  So we have #VC + #DB + #VC,
#VC + NMI + #VC, #VC + MCE + #VC, and even worse options.  So doing
the shift in a reliable way is not necessarily possible in a clean
way.

Let me contemplate.   And maybe produce some code soon.