Re: [PATCH 0/3] KVM: s390: Use ESCA instead of BSCA at VM init

[David Hildenbrand <david@redhat.com>]

On 14.05.25 18:34, 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 we will now 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.

Trying to remember vsie details ... I recall that for the vsie we never 
cared about the layout, because we simply pin+forward the given block, 
but disable any facility that would try de-referencing the vcpu 
pointers. So we only pin a single page.

pin_blocks() documents: "As we reuse the sca, the vcpu pointers 
contained in it are invalid. We must therefore not enable any facilities 
that access these pointers (e.g. SIGPIF)."


So I assume this change here will not affect (degrade) when being run as 
a nested hypervisor, right?

-- 
Cheers,

David / dhildenb