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

[Michael Roth]

On Thu, Jan 06, 2022 at 12:38:37PM -0600, Venu Busireddy wrote:
> On 2021-12-10 09:43:14 -0600, Brijesh Singh wrote:
> > From: Michael Roth <michael.roth@xxxxxxx>
> > 
> > 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@xxxxxxx>
> > Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> > ---
> >  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?

For me it was more about readability. E?X are well-defined as 32-bit
values, whereas regs->?x are longs. It seemed more readable to me to
have sev_cpuid_hv()/snp_cpuid() expect/return the actual native types,
and leave it up to the caller to cast/shift if necessary.

It also seems more robust for future re-use, since, for instance, if we
ever introduced another callsite that happened to already use u32 locally,
it seems like it would be a mess trying to setup up temp long* args or do
casts to pass them into these functions and then shift/cast them back just
so we could save a few lines at this particular callsite.

> 
> Venu