Re: [PATCH 3/3] x86/sev-es: Improve comments in and around __sev_es_ist_enter/exit()
From: Borislav Petkov <bp@alien8.de>
Date: 2021-02-17 18:01:58
Also in:
kvm, virtualization
On Wed, Feb 17, 2021 at 01:01:43PM +0100, Joerg Roedel wrote:
quoted hunk ↗ jump to hunk
From: Joerg Roedel <redacted> Better explain why this code is necessary and what it is doing. Signed-off-by: Joerg Roedel <redacted> --- 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