Re: [PATCH v2 33/35] x86/domain: implement domain_has_vuart()

[Denis Mukhin <dmkhn@xxxxxxxxx>]

On Friday, December 13th, 2024 at 12:45 PM, Stefano Stabellini 
<sstabellini@xxxxxxxxxx> wrote:

>
>
> On Fri, 13 Dec 2024, Roger Pau Monné wrote:
>
> > On Thu, Dec 05, 2024 at 08:42:03PM -0800, Denis Mukhin via B4 Relay wrote:
> >
> > > From: Denis Mukhin dmukhin@xxxxxxxx
> > >
> > > Add new emulation flag for virtual UART on x86 and plumb it through the 
> > > stack.
> > >
> > > This change enables NS8250 emulator initialization.
> > >
> > > Signed-off-by: Denis Mukhin dmukhin@xxxxxxxx
> > > ---
> > > tools/libs/light/libxl_x86.c | 6 +++++-
> > > tools/ocaml/libs/xc/xenctrl.ml | 1 +
> > > tools/ocaml/libs/xc/xenctrl.mli | 1 +
> > > tools/python/xen/lowlevel/xc/xc.c | 4 +---
> > > xen/arch/x86/domain.c | 8 +++++---
> > > xen/arch/x86/include/asm/domain.h | 7 ++++---
> > > xen/include/public/arch-x86/xen.h | 14 +++++++++++++-
> > > 7 files changed, 30 insertions(+), 11 deletions(-)
> > >
> > > diff --git a/tools/libs/light/libxl_x86.c b/tools/libs/light/libxl_x86.c
> > > index 
> > > a3164a3077fec7e1b81a34074894dc646954a49a..de5f05e18cb0671bb031b101b9a7159eb0fe0178
> > >  100644
> > > --- a/tools/libs/light/libxl_x86.c
> > > +++ b/tools/libs/light/libxl_x86.c
> > > @@ -8,7 +8,11 @@ int libxl__arch_domain_prepare_config(libxl__gc gc,
> > > {
> > > switch(d_config->c_info.type) {
> > > case LIBXL_DOMAIN_TYPE_HVM:
> > > - config->arch.emulation_flags = (XEN_X86_EMU_ALL & ~XEN_X86_EMU_VPCI);
> > > + config->arch.emulation_flags = XEN_X86_EMU_ALL;
> > > + config->arch.emulation_flags &= ~XEN_X86_EMU_VPCI;
> > > + / Virtual UART is selected at Xen build time */
> > > + config->arch.emulation_flags &= ~XEN_X86_EMU_VUART;
> > > +
> > > if (!libxl_defbool_val(d_config->b_info.u.hvm.pirq))
> > > config->arch.emulation_flags &= ~XEN_X86_EMU_USE_PIRQ;
> > > break;
> > > diff --git a/tools/ocaml/libs/xc/xenctrl.ml 
> > > b/tools/ocaml/libs/xc/xenctrl.ml
> > > index 
> > > 2690f9a92316b812ad3d3ff0e1c36823070adb4a..647239b3e55e88b00eb8e9773a5267894cbbae54
> > >  100644
> > > --- a/tools/ocaml/libs/xc/xenctrl.ml
> > > +++ b/tools/ocaml/libs/xc/xenctrl.ml
> > > @@ -47,6 +47,7 @@ type x86_arch_emulation_flags =
> > > | X86_EMU_PIT
> > > | X86_EMU_USE_PIRQ
> > > | X86_EMU_VPCI
> > > + | X86_EMU_VUART
> > >
> > > type x86_arch_misc_flags =
> > > | X86_MSR_RELAXED
> > > diff --git a/tools/ocaml/libs/xc/xenctrl.mli 
> > > b/tools/ocaml/libs/xc/xenctrl.mli
> > > index 
> > > febbe1f6ae3f10c5abe45eaa3c06a8a67d9ba268..4f5f64c786e83e8a0c3dd3cdb0460f7095de4a62
> > >  100644
> > > --- a/tools/ocaml/libs/xc/xenctrl.mli
> > > +++ b/tools/ocaml/libs/xc/xenctrl.mli
> > > @@ -41,6 +41,7 @@ type x86_arch_emulation_flags =
> > > | X86_EMU_PIT
> > > | X86_EMU_USE_PIRQ
> > > | X86_EMU_VPCI
> > > + | X86_EMU_VUART
> > >
> > > type x86_arch_misc_flags =
> > > | X86_MSR_RELAXED
> > > diff --git a/tools/python/xen/lowlevel/xc/xc.c 
> > > b/tools/python/xen/lowlevel/xc/xc.c
> > > index 
> > > 9feb12ae2b16e48cb5d0c3c45044ae226f152f2d..e54308956efc7061d58d2166ec9a95bc1dcd1781
> > >  100644
> > > --- a/tools/python/xen/lowlevel/xc/xc.c
> > > +++ b/tools/python/xen/lowlevel/xc/xc.c
> > > @@ -159,9 +159,7 @@ static PyObject *pyxc_domain_create(XcObject *self,
> > >
> > > #if defined (__i386) || defined(x86_64)
> > > if ( config.flags & XEN_DOMCTL_CDF_hvm )
> > > - config.arch.emulation_flags = XEN_X86_EMU_ALL &
> > > - ~(XEN_X86_EMU_VPCI |
> > > - XEN_X86_EMU_USE_PIRQ);
> > > + config.arch.emulation_flags = XEN_X86_EMU_HVM_ALLOWABLE;
> > > #elif defined (arm) || defined(aarch64)
> > > config.arch.gic_version = XEN_DOMCTL_CONFIG_GIC_NATIVE;
> > > #else
> > > diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
> > > index 
> > > c88d422a64544531c1e1058fa484364bb4277d1e..439da7adc92a3a8eb481075bf834da5f9670dd54
> > >  100644
> > > --- a/xen/arch/x86/domain.c
> > > +++ b/xen/arch/x86/domain.c
> > > @@ -752,10 +752,10 @@ static bool emulation_flags_ok(const struct domain 
> > > *d, uint32_t emflags)
> > > if ( is_hardware_domain(d) &&
> > > emflags != (X86_EMU_VPCI | X86_EMU_LAPIC | X86_EMU_IOAPIC) )
> > > return false;
> > > +
> > > + emflags &= ~X86_EMU_VUART;
> >
> > I think you want to allow X86_EMU_VUART only for domains created by
> > Xen itself, so X86_EMU_VUART can only be valid if system_state <
> > SYS_STATE_active.
> >
> > > if ( !is_hardware_domain(d) &&
> > > - /* HVM PIRQ feature is user-selectable. */
> > > - (emflags & ~X86_EMU_USE_PIRQ) !=
> > > - (X86_EMU_ALL & ~(X86_EMU_VPCI | X86_EMU_USE_PIRQ)) &&
> > > + xen_emflags_allowable(emflags) != XEN_X86_EMU_HVM_ALLOWABLE &&
> > > emflags != X86_EMU_LAPIC )
> > > return false;
> > > }
> > > @@ -806,6 +806,8 @@ int arch_domain_create(struct domain *d,
> > >
> > > emflags = config->arch.emulation_flags;
> > >
> > > + if ( IS_ENABLED(CONFIG_HAS_VUART_NS8250) && is_hvm_domain(d) )
> > > + emflags |= XEN_X86_EMU_VUART;
> >
> > Doesn't this need to be limited to domains created by Xen itself, as
> > otherwise it's the toolstack that should specify the XEN_X86_EMU_VUART
> > flag, and even then the recommendation would be to use the vUART from
> > QEMU?
>
>
> While I agree with you that this feature is really useful mostly for the
> domains created by Xen, as for those there is no other way to get early
> output, I think Denis has been also testing successfully this feature
> with PVH or HVM domains created by the toolstack.

I tested w/ HVM only for now.
PVH needs some more work as it does not have vPIC.

>
> I'll let you decide whether you want to expose the feature to xl created
> domains, but yes my understanding is that they already work with this
> series. One benefit would be that for PVH domains you could get early
> output without having to start QEMU, but I'll leave this to you.

TBH, I planned to enable xl support in the follow on series.