Re: [PATCH v8 22/40] x86/sev: move MSR-based VMGEXITs for CPUID to helper

Venu Busireddy
2022-01-06T18:39:27Z

On 2021-12-10 09:43:14 -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.
>
> Signed-off-by: Michael Roth <michael.roth@amd.com>
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> ---
> arch/x86/kernel/sev-shared.c | 84 +++++++++++++++++++++++++-----------
> 1 file changed, 58 insertions(+), 26 deletions(-)
>
> diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
> index 3aaef1a18ffe..d89481b31022 100644
> --- a/arch/x86/kernel/sev-shared.c
> +++ b/arch/x86/kernel/sev-shared.c
> @@ -194,6 +194,58 @@ 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, u32 subfunc, u32 *eax, u32 *ebx,
> + u32 *ecx, u32 *edx)
> +{
> + u64 val;
> +
> + if (eax) {
> + sev_es_wr_ghcb_msr(GHCB_CPUID_REQ(func, GHCB_CPUID_REQ_EAX));
> + VMGEXIT();
> + val = sev_es_rd_ghcb_msr();
> +
> + if (GHCB_RESP_CODE(val) != GHCB_MSR_CPUID_RESP)
> + return -EIO;
> +
> + *eax = (val >> 32);
> + }
> +
> + if (ebx) {
> + sev_es_wr_ghcb_msr(GHCB_CPUID_REQ(func, GHCB_CPUID_REQ_EBX));
> + VMGEXIT();
> + val = sev_es_rd_ghcb_msr();
> +
> + if (GHCB_RESP_CODE(val) != GHCB_MSR_CPUID_RESP)
> + return -EIO;
> +
> + *ebx = (val >> 32);
> + }
> +
> + if (ecx) {
> + sev_es_wr_ghcb_msr(GHCB_CPUID_REQ(func, GHCB_CPUID_REQ_ECX));
> + VMGEXIT();
> + val = sev_es_rd_ghcb_msr();
> +
> + if (GHCB_RESP_CODE(val) != GHCB_MSR_CPUID_RESP)
> + return -EIO;
> +
> + *ecx = (val >> 32);
> + }
> +
> + if (edx) {
> + sev_es_wr_ghcb_msr(GHCB_CPUID_REQ(func, GHCB_CPUID_REQ_EDX));
> + VMGEXIT();
> + val = sev_es_rd_ghcb_msr();
> +
> + if (GHCB_RESP_CODE(val) != GHCB_MSR_CPUID_RESP)
> + return -EIO;
> +
> + *edx = (val >> 32);
> + }
> +
> + return 0;
> +}
> +
> /*
> * Boot VC Handler - This is the first VC handler during boot, there is no GHCB
> * page yet, so it only supports the MSR based communication with the
> @@ -202,39 +254,19 @@ enum es_result sev_es_ghcb_hv_call(struct ghcb *ghcb, bool set_ghcb_msr,
> void __init do_vc_no_ghcb(struct pt_regs *regs, unsigned long exit_code)
> {
> unsigned int fn = lower_bits(regs->ax, 32);
> - unsigned long val;
> + u32 eax, ebx, ecx, edx;
>
> /* Only CPUID is supported via MSR protocol */
> if (exit_code != SVM_EXIT_CPUID)
> goto fail;
>
> - sev_es_wr_ghcb_msr(GHCB_CPUID_REQ(fn, GHCB_CPUID_REQ_EAX));
> - VMGEXIT();
> - val = sev_es_rd_ghcb_msr();
> - if (GHCB_RESP_CODE(val) != GHCB_MSR_CPUID_RESP)
> + if (sev_cpuid_hv(fn, 0, &eax, &ebx, &ecx, &edx))
> goto fail;
> - regs->ax = val >> 32;
>
> - sev_es_wr_ghcb_msr(GHCB_CPUID_REQ(fn, GHCB_CPUID_REQ_EBX));
> - VMGEXIT();
> - val = sev_es_rd_ghcb_msr();
> - if (GHCB_RESP_CODE(val) != GHCB_MSR_CPUID_RESP)
> - goto fail;
> - regs->bx = val >> 32;
> -
> - sev_es_wr_ghcb_msr(GHCB_CPUID_REQ(fn, GHCB_CPUID_REQ_ECX));
> - VMGEXIT();
> - val = sev_es_rd_ghcb_msr();
> - if (GHCB_RESP_CODE(val) != GHCB_MSR_CPUID_RESP)
> - goto fail;
> - regs->cx = val >> 32;
> -
> - sev_es_wr_ghcb_msr(GHCB_CPUID_REQ(fn, GHCB_CPUID_REQ_EDX));
> - VMGEXIT();
> - val = sev_es_rd_ghcb_msr();
> - if (GHCB_RESP_CODE(val) != GHCB_MSR_CPUID_RESP)
> - goto fail;
> - regs->dx = val >> 32;
> + regs->ax = eax;
> + regs->bx = ebx;
> + regs->cx = ecx;
> + regs->dx = edx;

What is the intent behind declaring e?x as local variables, instead
of passing the addresses of regs->?x to sev_cpuid_hv()? Is it to
prevent touching any of the regs->?x unless there is no error from
sev_cpuid_hv()? If so, wouldn't it be better to hide this logic from
the callers by declaring the local variables in sev_cpuid_hv() itself,
and moving the four "*e?x = (val >> 32);" statements there to the end
of the function (just before last the return)? With that change, the
callers can safely pass the addresses of regs->?x to do_vc_no_ghcb(),
knowing that the values will only be touched if there is no error?

Venu