Messages in this thread | | | Date | Wed, 17 Feb 2021 19:00:54 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH 3/3] x86/sev-es: Improve comments in and around __sev_es_ist_enter/exit() |
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On Wed, Feb 17, 2021 at 01:01:43PM +0100, Joerg Roedel wrote: > From: Joerg Roedel <jroedel@suse.de> > > Better explain why this code is necessary and what it is doing. > > Signed-off-by: Joerg Roedel <jroedel@suse.de> > --- > arch/x86/kernel/sev-es.c | 23 ++++++++++++++++------- > 1 file changed, 16 insertions(+), 7 deletions(-) > > diff --git a/arch/x86/kernel/sev-es.c b/arch/x86/kernel/sev-es.c > index 0df38b185d53..79241bc45f25 100644 > --- a/arch/x86/kernel/sev-es.c > +++ b/arch/x86/kernel/sev-es.c > @@ -127,14 +127,20 @@ static __always_inline bool on_vc_stack(unsigned long sp) > } > > /* > - * This function handles the case when an NMI is raised in the #VC exception > - * handler entry code. In this case, the IST entry for #VC must be adjusted, so > - * that any subsequent #VC exception will not overwrite the stack contents of the > - * interrupted #VC handler. > + * This function handles the case when an NMI is raised in the #VC > + * exception handler entry code, before the #VC handler has switched off > + * its IST stack. In this case, the IST entry for #VC must be adjusted, > + * so that any nested #VC exception will not overwrite the stack > + * contents of the interrupted #VC handler. > * > * The IST entry is adjusted unconditionally so that it can be also be > - * unconditionally adjusted back in sev_es_ist_exit(). Otherwise a nested > - * sev_es_ist_exit() call may adjust back the IST entry too early. > + * unconditionally adjusted back in __sev_es_ist_exit(). Otherwise a > + * nested sev_es_ist_exit() call may adjust back the IST entry too > + * early. > + * > + * The __sev_es_ist_enter() and __sev_es_ist_exit() functions always run > + * on the NMI IST stack, as they are only called from NMI handling code > + * right now. > */ > void noinstr __sev_es_ist_enter(struct pt_regs *regs) > { > @@ -143,7 +149,10 @@ void noinstr __sev_es_ist_enter(struct pt_regs *regs) > /* Read old IST entry */ > old_ist = __this_cpu_read(cpu_tss_rw.x86_tss.ist[IST_INDEX_VC]); > > - /* Make room on the IST stack */ > + /* > + * Make room on the IST stack - Reserve 8 bytes to store the old > + * IST entry. > + */ > if (on_vc_stack(regs->sp) && > !user_mode(regs) && > !from_syscall_gap(regs)) > --
Yah, and then we probably should simplify this __sev_es_ist_enter() function even more as it is not easy to grok.
For example, the ALIGN_DOWN(regs->sp, 8) is not really needed, right?
Also, both branches do "- sizeof(old_ist);" so you can just as well do it unconditionally.
And the sizeof(old_ist) is just a confusing way to write 8, right? We're 64-bit only so there's no need for that, I'd say.
And then you probably should change the comments from
/* Store old IST entry */
and
/* Set new IST entry */
to something like:
/* * If on the #VC IST stack, new_ist gets set to point one stack slot * further down from the #VC interrupt frame which has been pushed on * it during the first #VC exception entry. * * If not, simply the next slot on the #VC IST stack is set to point...
and here I'm not even sure why we're doing it?
The else branch, when we're not on the #VC stack, why are we doing
new_ist = old_ist - sizeof(old_ist);
?
I mean, if the NMI handler causes a #VC exception, it will simply run on the #VC IST stack so why do we have to do that - 8 thing at all?
Thx.
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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