Re: [Xen-devel] [PATCH v3 3/3] x86: Clean up the Xen MSR infrastructure

[Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>]

On Wed, 2018-09-12 at 11:23 +0100, Andrew Cooper wrote:
> On 12/09/18 10:46, Sergey Dyasli wrote:
> > On Wed, 2018-09-12 at 10:12 +0100, Andrew Cooper wrote:
> > > On 12/09/18 09:29, Sergey Dyasli wrote:
> > > > On Tue, 2018-09-11 at 19:56 +0100, Andrew Cooper wrote:
> > > > > Rename them to guest_{rd,wr}msr_xen() for consistency, and because 
> > > > > the _regs
> > > > > suffix isn't very appropriate.
> > > > > 
> > > > > Update them to take a vcpu pointer rather than presuming that they 
> > > > > act on
> > > > > current, and switch to using X86EMUL_* return values.
> > > > > 
> > > > > Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > > > > ---
> > > > > CC: Jan Beulich <JBeulich@xxxxxxxx>
> > > > > CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> > > > > CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > > > > CC: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
> > > > > 
> > > > > v3:
> > > > >  * Clean up after splitting the series.
> > > > > ---
> > > > >  xen/arch/x86/msr.c              |  6 ++----
> > > > >  xen/arch/x86/traps.c            | 29 +++++++++++++----------------
> > > > >  xen/include/asm-x86/processor.h |  4 ++--
> > > > >  3 files changed, 17 insertions(+), 22 deletions(-)
> > > > > 
> > > > > diff --git a/xen/arch/x86/msr.c b/xen/arch/x86/msr.c
> > > > > index cf0dc27..8f02a89 100644
> > > > > --- a/xen/arch/x86/msr.c
> > > > > +++ b/xen/arch/x86/msr.c
> > > > > @@ -156,8 +156,7 @@ int guest_rdmsr(const struct vcpu *v, uint32_t 
> > > > > msr, uint64_t *val)
> > > > >  
> > > > >          /* Fallthrough. */
> > > > >      case 0x40000200 ... 0x400002ff:
> > > > > -        ret = (rdmsr_hypervisor_regs(msr, val)
> > > > > -               ? X86EMUL_OKAY : X86EMUL_EXCEPTION);
> > > > > +        ret = guest_rdmsr_xen(v, msr, val);
> > > > >          break;
> > > > >  
> > > > >      default:
> > > > > @@ -277,8 +276,7 @@ int guest_wrmsr(struct vcpu *v, uint32_t msr, 
> > > > > uint64_t val)
> > > > >  
> > > > >          /* Fallthrough. */
> > > > >      case 0x40000200 ... 0x400002ff:
> > > > > -        ret = (wrmsr_hypervisor_regs(msr, val) == 1
> > > > > -               ? X86EMUL_OKAY : X86EMUL_EXCEPTION);
> > > > > +        ret = guest_wrmsr_xen(v, msr, val);
> > > > >          break;
> > > > >  
> > > > >      default:
> > > > > diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
> > > > > index 7c17806..3988753 100644
> > > > > --- a/xen/arch/x86/traps.c
> > > > > +++ b/xen/arch/x86/traps.c
> > > > > @@ -768,29 +768,25 @@ static void do_trap(struct cpu_user_regs *regs)
> > > > >            trapnr, trapstr(trapnr), regs->error_code);
> > > > >  }
> > > > >  
> > > > > -/* Returns 0 if not handled, and non-0 for success. */
> > > > > -int rdmsr_hypervisor_regs(uint32_t idx, uint64_t *val)
> > > > > +int guest_rdmsr_xen(const struct vcpu *v, uint32_t idx, uint64_t 
> > > > > *val)
> > > > >  {
> > > > > -    struct domain *d = current->domain;
> > > > > +    const struct domain *d = v->domain;
> > > > >      /* Optionally shift out of the way of Viridian architectural 
> > > > > MSRs. */
> > > > >      uint32_t base = is_viridian_domain(d) ? 0x40000200 : 0x40000000;
> > > > >  
> > > > >      switch ( idx - base )
> > > > >      {
> > > > >      case 0: /* Write hypercall page MSR.  Read as zero. */
> > > > > -    {
> > > > >          *val = 0;
> > > > > -        return 1;
> > > > > -    }
> > > > > +        return X86EMUL_OKAY;
> > > > >      }
> > > > >  
> > > > > -    return 0;
> > > > > +    return X86EMUL_EXCEPTION;
> > > > >  }
> > > > >  
> > > > > -/* Returns 1 if handled, 0 if not and -Exx for error. */
> > > > > -int wrmsr_hypervisor_regs(uint32_t idx, uint64_t val)
> > > > > +int guest_wrmsr_xen(struct vcpu *v, uint32_t idx, uint64_t val)
> > > > >  {
> > > > > -    struct domain *d = current->domain;
> > > > > +    struct domain *d = v->domain;
> > > > >      /* Optionally shift out of the way of Viridian architectural 
> > > > > MSRs. */
> > > > >      uint32_t base = is_viridian_domain(d) ? 0x40000200 : 0x40000000;
> > > > >  
> > > > > @@ -809,7 +805,7 @@ int wrmsr_hypervisor_regs(uint32_t idx, uint64_t 
> > > > > val)
> > > > >              gdprintk(XENLOG_WARNING,
> > > > >                       "wrmsr hypercall page index %#x unsupported\n",
> > > > >                       page_index);
> > > > > -            return 0;
> > > > > +            return X86EMUL_EXCEPTION;
> > > > >          }
> > > > >  
> > > > >          page = get_page_from_gfn(d, gmfn, &t, P2M_ALLOC);
> > > > > @@ -822,13 +818,13 @@ int wrmsr_hypervisor_regs(uint32_t idx, 
> > > > > uint64_t val)
> > > > >              if ( p2m_is_paging(t) )
> > > > >              {
> > > > >                  p2m_mem_paging_populate(d, gmfn);
> > > > > -                return -ERESTART;
> > > > > +                return X86EMUL_RETRY;
> > > > 
> > > > Previously -ERESTART would've been converted to X86EMUL_EXCEPTION. But
> > > > with this patch, X86EMUL_RETRY will actually be returned. I don't think
> > > > that callers can handle this situation.
> > > > 
> > > > E.g. the code from vmx_vmexit_handler():
> > > > 
> > > >     case EXIT_REASON_MSR_WRITE:
> > > >         switch ( hvm_msr_write_intercept(regs->ecx, msr_fold(regs), 1) )
> > > >         {
> > > >         case X86EMUL_OKAY:
> > > >             update_guest_eip(); /* Safe: WRMSR */
> > > >             break;
> > > > 
> > > >         case X86EMUL_EXCEPTION:
> > > >             hvm_inject_hw_exception(TRAP_gp_fault, 0);
> > > >             break;
> > > >         }
> > > >         break;
> > > 
> > > Hmm lovely, so it was broken before, but should be correct now.
> > > 
> > > RETRY has caused an entry to go onto the paging ring, which will pause
> > > the vcpu until a reply occurs, after which we will re-enter the guest
> > > without having moved RIP forwards, re-execute the wrmsr instruction, and
> > > this time succeed because the frame has been paged in.
> > 
> > Actually, the current VMX/SVM (but not PV) code does:
> > 
> >         switch ( wrmsr_hypervisor_regs(msr, msr_content) )
> >         {
> >         case -ERESTART:
> >             return X86EMUL_RETRY;
> > 
> > This code is removed in 1/3 patch but I wasn't CCed.
> 
> Ah right, in which case I need to temporarily transplant this switch
> into patch 1.  Given its only the PV side which is then broken, I can
> probably see about doing a bugfix for that.

With this being rebased on top of v4 1/3:

Reviewed-by: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>

-- 
Thanks,
Sergey
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