Re: [PATCH 4/4] KVM: MIPS: Implement console output hypercall

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On Mon, Feb 06, 2017 at 02:25:59PM +0100, Paolo Bonzini wrote:
> 
> 
> On 06/02/2017 11:46, James Hogan wrote:
> > Documentation/virtual/kvm/api.txt seems to suggest that
> > KVM_EXIT_HYPERCALL is obsolete. When it suggests using KVM_EXIT_MMIO,
> > does it simply mean the guest should use MMIO to some virtio device of
> > some sort rather than using hypercalls, or that the hypercall should
> > somehow be munged into the mmio exit information?
> 
> The former.

Okay, thanks.

> 
> But there are cases when using hypercalls is unavoidable.  In that case
> the trend is to use other exit reasons than KVM_EXIT_HYPERCALL, such as
> KVM_EXIT_PAPR_HCALL in PowerPC.  Feel free to add KVM_EXIT_MIPS_CONOUT
> or something like that.

Okay, that sounds sensible. The existing mips_paravirt_defconfig does
contain CONFIG_VIRTIO_CONSOLE=y though, so I'm thinking we may be able
to get away without this hypercall and without old paravirt guest
kernels becoming unusable.

David/Andreas: would you agree, or do you feel strongly that this
hypercall API should be kept? (with a different KVM exit reason)

> 
> How would you find the character device to write to in QEMU?

I imagine it'd need a custom character device driver in QEMU so it could
be wired up to stdio/pty or whatever using QEMU arguments. I've only
tested it with a test case in my own MIPS KVM test suite so far though.

Cheers
James

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