Re: [PATCH v3 39/39] xen/riscv: introduce IMSIC h/w interrupt file attaching to vcpu

[Baptiste Le Duc <baptiste.le-duc@xxxxxxxxxx>]

> Introduce imsic_vsfile_attach() to give a vCPU a working guest
> interrupt file.
> 
> A guest (VS) interrupt file has to be backed by one of the hardware
> guest interrupt files of the pCPU the vCPU runs on, so that pCPU has to
> be known first. arch_vcpu_create() is therefore not a suitable place:
> the pCPU assigned to a vCPU can still change before the vCPU is first
> scheduled, which would mean taking another file and remapping the
> guest's IMSIC page to it. Call imsic_vsfile_attach() from
> continue_new_vcpu() instead, i.e. when the vCPU is scheduled for the
> first time; as that isn't an __init context, neither is
> imsic_vsfile_attach().
> 
> Giving a vCPU its first file shares most of the steps with moving it to
> a new one on migration, so move the body of imsic_migrate_vcpu() to
> imsic_vsfile_move() and use it for both. Its first_attach argument skips
> the part which moves the interrupt state out of the old file: the vCPU
> has never run at that point, so the freshly acquired and zeroed file is
> all the interrupt state it has, and hstatus.VGEIN can be pointed at it
> right away.
> 
> vAPLIC target registers are reconfigured on the first attach as well: a
> target the guest writes for a vCPU which has no h/w guest interrupt file
> yet only updates the stored copy, which reaches the h/w at this point.
> 
Nit: I didn't quite understand this paragraph, "only updates"
leaves out that the h/w APLIC write is skipped, because there is no file
to point the target at yet. Maybe something like:

    The guest may program a vAPLIC TARGET register for a vCPU which has
    not run yet. That vCPU has no h/w guest interrupt file yet, so
    vaplic_mmio_write() only stores the value. imsic_vsfile_move() then
    writes it to the h/w APLIC on the first attach, pointing to the newly
    acquired file.

Reviewed-by: Baptiste Le Duc <baptiste.le-duc@xxxxxxxxxx>

-- 
Baptiste Le Duc <baptiste.le-duc@xxxxxxxxxx>