Re: [PATCH v9 22/43] x86/sev: Move MSR-based VMGEXITs for CPUID to helper

Borislav Petkov
2022-02-03T13:59:47Z

On Fri, Jan 28, 2022 at 11:17:43AM -0600, Brijesh Singh wrote:
> From: Michael Roth <michael.roth@amd.com>
>
> This code will also be used later for SEV-SNP-validated CPUID code in
> some cases, so move it to a common helper.

Suggested-by: Sean Christopherson <seanjc@google.com>

unless he really doesn't want to be the "suggestor" :)

> Signed-off-by: Michael Roth <michael.roth@amd.com>
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> ---
> arch/x86/kernel/sev-shared.c | 62 +++++++++++++++++++++---------------
> 1 file changed, 36 insertions(+), 26 deletions(-)
>
> diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
> index 3aaef1a18ffe..633f1f93b6e1 100644
> --- a/arch/x86/kernel/sev-shared.c
> +++ b/arch/x86/kernel/sev-shared.c
> @@ -194,6 +194,36 @@ enum es_result sev_es_ghcb_hv_call(struct ghcb *ghcb, bool set_ghcb_msr,
> return verify_exception_info(ghcb, ctxt);
> }
>
> +static int __sev_cpuid_hv(u32 func, int reg_idx, u32 *reg)
> +{
> + u64 val;
> +
> + if (!reg)
> + return 0;

I don't know what that's supposed to catch? In case callers are
interested only in some subset of the CPUID leaf?

Meh, I guess...

> + sev_es_wr_ghcb_msr(GHCB_CPUID_REQ(func, reg_idx));
> + VMGEXIT();
> + val = sev_es_rd_ghcb_msr();
> + if (GHCB_RESP_CODE(val) != GHCB_MSR_CPUID_RESP)
> + return -EIO;
> +
> + *reg = (val >> 32);
> +
> + return 0;
> +}
> +
> +static int sev_cpuid_hv(u32 func, u32 *eax, u32 *ebx, u32 *ecx, u32 *edx)
> +{
> + int ret;
> +
> + ret = __sev_cpuid_hv(func, GHCB_CPUID_REQ_EAX, eax);
> + ret = ret ? : __sev_cpuid_hv(func, GHCB_CPUID_REQ_EBX, ebx);
> + ret = ret ? : __sev_cpuid_hv(func, GHCB_CPUID_REQ_ECX, ecx);
> + ret = ret ? : __sev_cpuid_hv(func, GHCB_CPUID_REQ_EDX, edx);

You can format that this way:

ret = __sev_cpuid_hv(func, GHCB_CPUID_REQ_EAX, eax);
ret = ret ? : __sev_cpuid_hv(func, GHCB_CPUID_REQ_EBX, ebx);
ret = ret ? : __sev_cpuid_hv(func, GHCB_CPUID_REQ_ECX, ecx);
ret = ret ? : __sev_cpuid_hv(func, GHCB_CPUID_REQ_EDX, edx);

and then it is visible at a quick glance what this does, due to the
regularity.

--
Regards/Gruss,
Boris.