Re: [RFC PATCH v2] x86/sev: enforce RIP-relative accesses in early SEV/SME code

[Kevin Loughlin]

On Mon, Jan 15, 2024 at 7:53 AM Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:
>
> On 1/11/24 16:36, Kevin Loughlin wrote:
> >
> > @@ -61,33 +66,34 @@ static __maybe_unused __always_inline bool amd_cc_platform_vtom(enum cc_attr att
> >   static bool noinstr amd_cc_platform_has(enum cc_attr attr)
> >   {
> >   #ifdef CONFIG_AMD_MEM_ENCRYPT
> > +     const u64 sev_status_fixed_up = sev_get_status_fixup();
>
> Why not also have a variable for sme_me_mask?

`sme_get_me_mask_fixup()` is only used on certain conditional paths in
the calculation of the return value (therefore, a max of 1 times per
invocation of `amd_cc_platform_has()`). As such, calling
`sme_get_me_mask()` unconditionally at the beginning of the function
would be unnecessary for some invocations of `amd_cc_platform_has()`.
In contrast, the sev_status is needed on every invocation of
`amd_cc_platform_has()`. Additionally, the `sev_get_status_fixup()`
result is potentially used multiple times in the same invocation of
`amd_cc_platform_has()`, motivating the use of a local variable.

> > @@ -130,6 +130,7 @@ static unsigned long __head sme_postprocess_startup(struct boot_params *bp, pmdv
> >   {
> >       unsigned long vaddr, vaddr_end;
> >       int i;
> > +     const u64 sme_me_mask_fixed_up = sme_get_me_mask_fixup();
>
> Should be the first variable listed given the length of the line.

I will incorporate this and all other stylistic changes that you
mentioned in v3.

> > @@ -110,8 +115,9 @@ static void __noreturn sev_es_terminate(unsigned int set, unsigned int reason)
> >   static u64 get_hv_features(void)
> >   {
> >       u64 val;
> > +     const u16 *ghcb_version_ptr = (const u16 *) GET_RIP_RELATIVE_PTR(ghcb_version);
>
> Is this one really needed? The ghcb_version variable isn't referenced
> before fixup, right? It's referenced in both decompression and early boot,
> but I didn't think a fixup is needed.

You're right; it looks like we do *not* need the fixup for
ghcb_version in both instances that you identified. I will remove
these particular fixups in v3.

Thanks!