On Fri, Sep 29, 2023 at 08:29:08AM -0300, Daniel Henrique Barboza wrote: > > > On 9/29/23 07:46, Daniel P. Berrangé wrote: > > On Tue, Sep 26, 2023 at 04:49:44PM -0300, Daniel Henrique Barboza wrote: > > > Based-on: 20230926183109.165878-1-dbarb...@ventanamicro.com > > > ("[PATCH 0/2] riscv: add extension properties for all cpus") > > > > > > Hi, > > > > > > These patches implements the base profile support for qemu-riscv and the > > > first profile, RVA22U64. > > > > > > As discussed in this thread [1] we're aiming for a flag that enables all > > > mandatory extensions of a profile. Optional extensions were left behind > > > and must be enabled by hand if desired. Since this is the first profile > > > we're adding, we'll need to add the base framework as well. > > > > > > The RVA22U64 profile was chosen because qemu-riscv implements all its > > > extensions, both mandatory and optional. That includes 'zicntr' and > > > 'zihpm', which we support for awhile but aren't adverting to userspace. > > > > > > Other design decisions made: > > > > > > - disabling a profile flag does nothing, i.e. we won't mass disable > > > mandatory extensions of the rva22U64 profile if the user sets > > > rva22u64=false; > > > > Why shouldn't this be allowed ? > > > > IIUC, a profile is syntactic sugar for a group of features. If > > we can disable individual features explicitly, why should we > > not allow use of the profile as sugar to disable them en-mass ? > > In theory there's no harm in allowing mass disabling of extensions but, given > it's a whole profile, we would end up disabling most/all CPU extensions and > the guest would do nothing.
True, that is just user error though. They could disable a profile and then manually re-enable individual features, and thus get a working system. > There is a thread in the ML: > > https://lore.kernel.org/qemu-riscv/cabjz62nyvnu4z1qmcg7myjkgg_9ywxjufhhwjmoqep6unrr...@mail.gmail.com/ > > Where we discussed the possibility of having a minimal CPU extension set. We > didn't > reach a consensus because the definition of "minimal CPU extension set" vary > between > OSes (Linux requires IMAFD, FreeBSD might require something differ). > > Assuming we reach a consensus on what a minimal set is, we could allow > disabling mass > extensions via probile but keeping this minimal set, for example. At very > least we > shouldn't allow users to disable 'I' because that would kill the CPU, so > RV64I is > the minimum set that I would assume for now. I'd probably just call that user error too. > > > > TL;DR: feature groups are pretty error prone if more than > > one is listed by the user, or they're combined with individual > > features. > > > > > > > > - profile support for vendor CPUs consists into checking if the CPU > > > happens to have the mandatory extensions required for it. In case it > > > doesn't we'll error out. This is done to follow the same prerogative > > > we always had of not allowing extensions being enabled for vendor > > > CPUs; > > > > Why shouldn't this be allowed ? > > There's no technical reason to not allow it. The reason it's forbid is to be > closer to what the real hardware would do. E.g. the real hardware doesn't > allow > users to enable Vector if the hardware doesn't support it. Vendor CPUs also > has > a privileged spec restriction as well, so if a CPU is running in an older spec > it can't enable extensions that were added later. Real hardware is constrained in not being able to invent arbitrary new features on chip. Virtual machines are not constrained, so I don't think the inability of hardware todo this, is an especially strong reason to limit software emulation. What I don't like about this, is that (IIUC) the '$profile=on' option now has different semantics depending on what CPU it is used with. ie using it with a vendor CPU, $profile=on becomes an assertion that the vendor CPU contains all the features needed to satisfy $profile. It won't enable/disable anything, just check it is present. With a non-vendor CPU, using $profile=on becomes a mechanism to force enable all the features needed to satisfy $profile, there is no mechanism to just check for presence. Having two different semantics for the same syntax is generally considered bad design practice. This points towards supporting a tri-state, not boolean. $profile=check for validation only, and $profile=on for force enablement. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|