Re: [PATCH] x86/vmce: Dispatch vmce_{rd,wr}msr() from guest_{rd,wr}msr()

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, Jul 22, 2020 at 11:18:09AM +0100, Andrew Cooper wrote:
> ... rather than from the default clauses of the PV and HVM MSR handlers.
> 
> This means that we no longer take the vmce lock for any unknown MSR, and
> accesses to architectural MCE banks outside of the subset implemented for the
> guest no longer fall further through the unknown MSR path.
> 
> With the vmce calls removed, the hvm alternative_call()'s expression can be
> simplified substantially.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

LGTM, I just have one question below regarding the ranges.

Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

> ---
> CC: Jan Beulich <JBeulich@suse.com>
> CC: Wei Liu <wl@xen.org>
> CC: Roger Pau Monné <roger.pau@citrix.com>
> ---
>  xen/arch/x86/hvm/hvm.c         | 16 ++--------------
>  xen/arch/x86/msr.c             | 16 ++++++++++++++++
>  xen/arch/x86/pv/emul-priv-op.c | 15 ---------------
>  3 files changed, 18 insertions(+), 29 deletions(-)
> 
> diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
> index 5bb47583b3..a9d1685549 100644
> --- a/xen/arch/x86/hvm/hvm.c
> +++ b/xen/arch/x86/hvm/hvm.c
> @@ -3560,13 +3560,7 @@ int hvm_msr_read_intercept(unsigned int msr, uint64_t *msr_content)
>           break;
>  
>      default:
> -        if ( (ret = vmce_rdmsr(msr, msr_content)) < 0 )
> -            goto gp_fault;
> -        /* If ret == 0 then this is not an MCE MSR, see other MSRs. */
> -        ret = ((ret == 0)
> -               ? alternative_call(hvm_funcs.msr_read_intercept,
> -                                  msr, msr_content)
> -               : X86EMUL_OKAY);
> +        ret = alternative_call(hvm_funcs.msr_read_intercept, msr, msr_content);
>          break;
>      }
>  
> @@ -3696,13 +3690,7 @@ int hvm_msr_write_intercept(unsigned int msr, uint64_t msr_content,
>          break;
>  
>      default:
> -        if ( (ret = vmce_wrmsr(msr, msr_content)) < 0 )
> -            goto gp_fault;
> -        /* If ret == 0 then this is not an MCE MSR, see other MSRs. */
> -        ret = ((ret == 0)
> -               ? alternative_call(hvm_funcs.msr_write_intercept,
> -                                  msr, msr_content)
> -               : X86EMUL_OKAY);
> +        ret = alternative_call(hvm_funcs.msr_write_intercept, msr, msr_content);
>          break;
>      }
>  
> diff --git a/xen/arch/x86/msr.c b/xen/arch/x86/msr.c
> index 22f921cc71..ca4307e19f 100644
> --- a/xen/arch/x86/msr.c
> +++ b/xen/arch/x86/msr.c
> @@ -227,6 +227,14 @@ int guest_rdmsr(struct vcpu *v, uint32_t msr, uint64_t *val)
>          *val = msrs->misc_features_enables.raw;
>          break;
>  
> +    case MSR_IA32_MCG_CAP     ... MSR_IA32_MCG_CTL:      /* 0x179 -> 0x17b */
> +    case MSR_IA32_MCx_CTL2(0) ... MSR_IA32_MCx_CTL2(31): /* 0x280 -> 0x29f */
> +    case MSR_IA32_MCx_CTL(0)  ... MSR_IA32_MCx_MISC(31): /* 0x400 -> 0x47f */

Where do you get the ranges from 0 to 31? It seems like the count
field in the CAP register is 8 bits, which could allow for up to 256
banks?

I'm quite sure this would then overlap with other MSRs?

> +    case MSR_IA32_MCG_EXT_CTL:                           /* 0x4d0 */
> +        if ( vmce_rdmsr(msr, val) < 0 )
> +            goto gp_fault;
> +        break;
> +
>      case MSR_X2APIC_FIRST ... MSR_X2APIC_LAST:
>          if ( !is_hvm_domain(d) || v != curr )
>              goto gp_fault;
> @@ -436,6 +444,14 @@ int guest_wrmsr(struct vcpu *v, uint32_t msr, uint64_t val)
>          break;
>      }
>  
> +    case MSR_IA32_MCG_CAP     ... MSR_IA32_MCG_CTL:      /* 0x179 -> 0x17b */
> +    case MSR_IA32_MCx_CTL2(0) ... MSR_IA32_MCx_CTL2(31): /* 0x280 -> 0x29f */
> +    case MSR_IA32_MCx_CTL(0)  ... MSR_IA32_MCx_MISC(31): /* 0x400 -> 0x47f */
> +    case MSR_IA32_MCG_EXT_CTL:                           /* 0x4d0 */
> +        if ( vmce_wrmsr(msr, val) < 0 )
> +            goto gp_fault;
> +        break;
> +
>      case MSR_X2APIC_FIRST ... MSR_X2APIC_LAST:
>          if ( !is_hvm_domain(d) || v != curr )
>              goto gp_fault;
> diff --git a/xen/arch/x86/pv/emul-priv-op.c b/xen/arch/x86/pv/emul-priv-op.c
> index 254da2b849..f14552cb4b 100644
> --- a/xen/arch/x86/pv/emul-priv-op.c
> +++ b/xen/arch/x86/pv/emul-priv-op.c
> @@ -855,8 +855,6 @@ static int read_msr(unsigned int reg, uint64_t *val,
>  
>      switch ( reg )
>      {
> -        int rc;
> -
>      case MSR_FS_BASE:
>          if ( is_pv_32bit_domain(currd) )
>              break;
> @@ -955,12 +953,6 @@ static int read_msr(unsigned int reg, uint64_t *val,
>          }
>          /* fall through */
>      default:
> -        rc = vmce_rdmsr(reg, val);
> -        if ( rc < 0 )
> -            break;
> -        if ( rc )
> -            return X86EMUL_OKAY;
> -        /* fall through */
>      normal:

We could remove the 'normal' label and just use the default one
instead.

Thanks, Roger.