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SubjectRe: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP from userspace)
Hi Andy,

On Mon, Apr 27, 2020 at 10:37:41AM -0700, Andy Lutomirski wrote:
> 1. Use IST for #VC and deal with all the mess that entails.

With the removal of IST shifting I wonder what you would suggest on how
to best implement an NMI-safe IST handler with nesting support.

My current plan is to implement an IST handler which switches itself off
the IST stack as soon as possible, freeing it for re-use.

The flow would be roughly like this upon entering the handler;

build_pt_regs();

RSP = pt_regs->sp;

if (RSP in VC_IST_stack)
error("unallowed nesting")

if (RSP in current_kernel_stack)
RSP = round_down_to_8(RSP)
else
RSP = current_top_of_stack() // non-ist kernel stack

copy_pt_regs(pt_regs, RSP);
switch_stack_to(RSP);

To make this NMI safe, the NMI handler needs some logic too. Upon
entering NMI, it needs to check the return RSP, and if it is in the #VC
IST stack, it must do the above flow by itself and update the return RSP
and RIP. It needs to take into account the case when PT_REGS is not
fully populated on the return side.

Alternativly the NMI handler could safe/restore the contents of the #VC
IST stack or just switch to a special #VC-in-NMI IST stack.

All in all it could get complicated, and imho shift_ist would have been
simpler, but who am I anyway...

Or maybe you have a better idea how to implement this, so I'd like to
hear your opinion first before I spend too many days implementing
something.

Regards,

Joerg

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