Re: [PATCH v3 4/8] s390/sclp: read sccb from mem based on sccb length

[Janosch Frank]

On 6/19/20 12:22 AM, Collin Walling wrote:
> The header of the SCCB contains the actual length of the SCCB. Instead
> of using a static 4K size, let's allow for a variable size determined
> by the value set in the header. The proper checks are already in place
> to ensure the SCCB length is sufficent to store a full response, and
> that the length does not cross any explicitly-set boundaries.
> 
> Signed-off-by: Collin Walling <walling@linux.ibm.com>
> Reviewed-by: Thomas Huth <thuth@redhat.com>

I wonder if this brings a measureable performance penalty with it for
protected guests. It's another ioctl to move the remaining bytes from
KVM to QEMU. On the other hand it's only sclp...

Reviewed-by: Janosch Frank <frankja@linux.ibm.com>

> ---
>  hw/s390x/sclp.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/hw/s390x/sclp.c b/hw/s390x/sclp.c
> index 0710138f91..772b7b3b01 100644
> --- a/hw/s390x/sclp.c
> +++ b/hw/s390x/sclp.c
> @@ -256,9 +256,8 @@ int sclp_service_call_protected(CPUS390XState *env, 
> uint64_t sccb,
>      SCLPDevice *sclp = get_sclp_device();
>      SCLPDeviceClass *sclp_c = SCLP_GET_CLASS(sclp);
>      SCCB work_sccb;
> -    hwaddr sccb_len = sizeof(SCCB);
>  
> -    s390_cpu_pv_mem_read(env_archcpu(env), 0, &work_sccb, sccb_len);
> +    s390_cpu_pv_mem_read(env_archcpu(env), 0, &work_sccb, 
> sizeof(SCCBHeader));
>  
>      if (!sclp_command_code_valid(code)) {
>          work_sccb.h.response_code = 
> cpu_to_be16(SCLP_RC_INVALID_SCLP_COMMAND);
> @@ -269,6 +268,9 @@ int sclp_service_call_protected(CPUS390XState *env, 
> uint64_t sccb,
>          goto out_write;
>      }
>  
> +    s390_cpu_pv_mem_read(env_archcpu(env), 0, &work_sccb,
> +                         be16_to_cpu(work_sccb.h.length));
> +
>      sclp_c->execute(sclp, &work_sccb, code);
>  out_write:
>      s390_cpu_pv_mem_write(env_archcpu(env), 0, &work_sccb,
> @@ -283,8 +285,6 @@ int sclp_service_call(CPUS390XState *env, uint64_t sccb, 
> uint32_t code)
>      SCLPDeviceClass *sclp_c = SCLP_GET_CLASS(sclp);
>      SCCB work_sccb;
>  
> -    hwaddr sccb_len = sizeof(SCCB);
> -
>      /* first some basic checks on program checks */
>      if (env->psw.mask & PSW_MASK_PSTATE) {
>          return -PGM_PRIVILEGED;
> @@ -302,7 +302,7 @@ int sclp_service_call(CPUS390XState *env, uint64_t sccb, 
> uint32_t code)
>       * from playing dirty tricks by modifying the memory content after
>       * the host has checked the values
>       */
> -    cpu_physical_memory_read(sccb, &work_sccb, sccb_len);
> +    cpu_physical_memory_read(sccb, &work_sccb, sizeof(SCCBHeader));
>  
>      /* Valid sccb sizes */
>      if (be16_to_cpu(work_sccb.h.length) < sizeof(SCCBHeader)) {
> @@ -318,6 +318,9 @@ int sclp_service_call(CPUS390XState *env, uint64_t sccb, 
> uint32_t code)
>          goto out_write;
>      }
>  
> +    /* the header contains the actual length of the sccb */
> +    cpu_physical_memory_read(sccb, &work_sccb, 
> be16_to_cpu(work_sccb.h.length));
> +
>      sclp_c->execute(sclp, &work_sccb, code);
>  out_write:
>      cpu_physical_memory_write(sccb, &work_sccb,
>

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