Re: [PATCH kernel v2] x86/compressed/64: reduce #VC nesting for intercepted CPUID for SEV-SNP guest

[Alexey Kardashevskiy]

On 30/9/23 17:17, Borislav Petkov wrote:
> On Tue, Sep 26, 2023 at 02:05:26PM +1000, Alexey Kardashevskiy wrote:
> > vc_raw_handle_exception #1: exit_code 72 (CPUID) eax d ecx 1
> > We lock the main GHCB and while it is locked we get to
>
> Please use passive voice in your commit message: no "we" or "I", etc,
> and describe your changes in imperative mood.
>
> Also, pls read section "2) Describe your changes" in
> Documentation/process/submitting-patches.rst for more details.
>
> Also, see section "Changelog" in
> Documentation/process/maintainer-tip.rst
>
> Bottom line is: personal pronouns are ambiguous in text, especially with
> so many parties/companies/etc developing the kernel so let's avoid them
> please.
>
> > snp_cpuid_postprocess() which executes "rdmsr" of MSR_IA32_XSS==0xda0 which
> > triggers:
> >
> > vc_raw_handle_exception #2: exit_code 7c (MSR) ecx da0
> > Here we lock the backup ghcb.
> >
> > And then PMC NMI comes which cannot complete as there is no GHCB page left
> > to use:
> >
> > CPU: 5 PID: 566 Comm: touch Not tainted 6.5.0-rc2-aik-ad9c-g7413e71d3dcf-dirty #27
> > Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS unknown unknown
> > Call Trace:
> >   <NMI>
> >   dump_stack_lvl+0x44/0x60
> >   panic+0x222/0x310
> >   ____sev_get_ghcb+0x21e/0x220
> >   __sev_es_nmi_complete+0x28/0xf0
> >   exc_nmi+0x1ac/0x1c0
> >   end_repeat_nmi+0x16/0x67
> > ...
> >   </NMI>
> >   <TASK>
> >   vc_raw_handle_exception+0x9e/0x2c0
> >   kernel_exc_vmm_communication+0x4d/0xa0
> >   asm_exc_vmm_communication+0x31/0x60
> > RIP: 0010:snp_cpuid+0x2ad/0x420
>
> Drop that splat like we talked.
>
> > +/* Paravirt SEV-ES rdmsr which avoids extra #VC event */
> > +#define rdmsr_safe_GHCB(msr, low, high, ghcb, ctxt) ({				\
> > +	int __ret;								\
> > +										\
> > +	ghcb_set_rcx((ghcb), (msr));						\
> > +	__ret = sev_es_ghcb_hv_call((ghcb), (ctxt), SVM_EXIT_MSR, 0, 0);	\
> > +	if (__ret == ES_OK) {							\
> > +		low  = (ghcb)->save.rax;					\
> > +		high = (ghcb)->save.rdx;					\
> > +		/* Invalidate qwords for likely another following GHCB call */	\
> > +		vc_ghcb_invalidate(ghcb);					\
> > +	}									\
> > +	__ret; })
> > +
>
> First of all, this should be a function, not a macro.

Ingo says different, who wins? :)

> Then, it should be defined only in sev-shared.c for now.

sev-shared.c makes me sad. Including .c is not ... nice, I would avoid 
adding stuff to it at any cost.

> Furthermore, it should not be called "rdmsr" or so but something like
>
> ghcb_prot_read_msr()
>
> or so to denote that it is using the GHCB protocol to read the MSR. I'm
> sure it'll gain more users with time.

What is "prot" going to signify?

And what about Tom's "x86/sev: Fix SNP CPUID requests to the 
hypervisor", are you taking that one or I have to repost this one and 
the Tom's patch?


--
Alexey