Re: [PATCH v3 12/19] target/riscv: add KVM specific MISA properties

[Andrew Jones]

On Thu, Jun 22, 2023 at 10:56:53AM -0300, Daniel Henrique Barboza wrote:
> Using all TCG user properties in KVM is tricky. First because KVM
> supports only a small subset of what TCG provides, so most of the
> cpu->cfg flags do nothing for KVM.
> 
> Second, and more important, we don't have a way of telling if any given
> value is an user input or not. For TCG this has a small impact since we
> just validating everything and error out if needed. But for KVM it would
> be good to know if a given value was set by the user or if it's a value
> already provided by KVM. Otherwise we don't know how to handle failed
> kvm_set_one_regs() when writing the configurations back.
> 
> These characteristics make it overly complicated to use the same user
> facing flags for both KVM and TCG. A simpler approach is to create KVM
> specific properties that have specialized logic, forking KVM and TCG use
> cases for those cases only. Fully separating KVM/TCG properties is
> unneeded at this point - in fact we want the user experience to be as
> equal as possible, regardless of the acceleration chosen.
> 
> We'll start this fork with the MISA properties, adding the MISA bits
> that the KVM driver currently supports. A new KVMCPUConfig type is
> introduced. It'll hold general information about an extension. For MISA
> extensions we're going to use the newly created misa_ext_infos[] to
> populate their name and description. 'offset' holds the MISA bit (RVA,
> RVC, ...). We're calling it 'offset' instead of 'misa_bit' because this
> same KVMCPUConfig struct will be used to multi-letter extensions later
> on.
> 
> This new type also holds a 'user_set' flag. This flag will be set when
> the user set an option that's different than what is already configured
> in the host, requiring KVM intervention to write the regs back during
> kvm_arch_init_vcpu(). Similar mechanics will be implemented for
> multi-letter extensions as well.
> 
> There is no need to duplicate more code than necessary, so we're going
> to use the existing kvm_riscv_init_user_properties() to add the KVM
> specific properties. Any code that is adding a TCG user prop is then
> changed slightly to verify first if there's a KVM prop with the same
> name already added.
> 
> Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
> Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
> ---
>  target/riscv/cpu.c | 10 ++++++
>  target/riscv/kvm.c | 78 ++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 88 insertions(+)
>

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>