Re: [PATCH v5] KVM: s390: Use ESCA instead of BSCA at VM init

[Janosch Frank <frankja@linux.ibm.com>]

On 6/3/25 6:35 PM, Christoph Schlameuss wrote:
> All modern IBM Z and Linux One machines do offer support for the
> Extended System Control Area (ESCA). The ESCA is available since the
> z114/z196 released in 2010.
> KVM needs to allocate and manage the SCA for guest VMs. Prior to this
> change the SCA was setup as Basic SCA only supporting a maximum of 64
> vCPUs when initializing the VM. With addition of the 65th vCPU the SCA
> was needed to be converted to a ESCA.
> 
> Instead of allocating a BSCA and upgrading it for PV or when adding the
> 65th cpu we can always allocate the ESCA directly upon VM creation
> simplifying the code in multiple places as well as completely removing
> the need to convert an existing SCA.
> 
> In cases where the ESCA is not supported (z10 and earlier) the use of
> the SCA entries and with that SIGP interpretation are disabled for VMs.
> This increases the number of exits from the VM in multiprocessor
> scenarios and thus decreases performance.
> The same is true for VSIE where SIGP is currently disabled and thus no
> SCA entries are used.
> 
> The only downside of the change is that we will always allocate 4 pages
> for a 248 cpu ESCA instead of a single page for the BSCA per VM.
> In return we can delete a bunch of checks and special handling depending
> on the SCA type as well as the whole BSCA to ESCA conversion.
> 
> With that behavior change we are no longer referencing a bsca_block in
> kvm->arch.sca. This will always be esca_block instead.
> By specifying the type of the sca as esca_block we can simplify access
> to the sca and get rid of some helpers while making the code clearer.
> 
> KVM_MAX_VCPUS is also moved to kvm_host_types to allow using this in
> future type definitions.
> 

Thanks for taking care of this, it makes the code a lot nicer to read.

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