Re: [V10 PATCH 11/23] PVH xen: support invalid op emulation for PVH

[George Dunlap <george.dunlap@eu.citrix.com>]

On 08/08/13 02:49, Mukesh Rathor wrote:
> On Wed, 7 Aug 2013 12:29:13 +0100
> George Dunlap <George.Dunlap@eu.citrix.com> wrote:
>
>> On Wed, Jul 24, 2013 at 2:59 AM, Mukesh Rathor
>> <mukesh.rathor@oracle.com> wrote:
>>> This patch supports invalid op emulation for PVH by calling
>>> appropriate copy macros and and HVM function to inject PF.
>>>
>>> Signed-off-by: Mukesh Rathor <mukesh.rathor@oracle.com>
>>> Reviewed-by: Jan Beulich <jbeulich@suse.com>
>>> ---
>>>   xen/arch/x86/traps.c        |   17 ++++++++++++++---
>>>   xen/include/asm-x86/traps.h |    1 +
>>>   2 files changed, 15 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
>>> index 378ef0a..a3ca70b 100644
>>> --- a/xen/arch/x86/traps.c
>>> +++ b/xen/arch/x86/traps.c
>>> @@ -459,6 +459,11 @@ static void instruction_done(
>>>       struct cpu_user_regs *regs, unsigned long eip, unsigned int
>>> bpmatch) {
>>>       regs->eip = eip;
>>> +
>>> +    /* PVH fixme: debug trap below */
>>> +    if ( is_pvh_vcpu(current) )
>>> +        return;
>> What exactly does this comment mean?  Do you mean, "FIXME: Make debug
>> trapping work for PVH guests"?  (i.e., this functionality will be
>> implemented later?)
> Correct, future work. Look at what the db trap is doing and make
> it work for PVH if it doesn't already.
>
>>> +
>>>       regs->eflags &= ~X86_EFLAGS_RF;
>>>       if ( bpmatch || (regs->eflags & X86_EFLAGS_TF) )
>>>       {
>>> @@ -913,7 +918,7 @@ static int emulate_invalid_rdtscp(struct
>>> cpu_user_regs *regs) return EXCRET_fault_fixed;
>>>   }
>>>
>>> -static int emulate_forced_invalid_op(struct cpu_user_regs *regs)
>>> +int emulate_forced_invalid_op(struct cpu_user_regs *regs)
>> Why make this non-static?  No one is using this in this patch.  If a
>> later patch needs it, you should make it non-static there, so we can
>> decide at that point if making it non-static is merited or not.
> Sigh! Originally, it was that way, but then to keep that patch from
> getting too big, it got moved here after few versions. We are making
> emulation available for outside the PV, ie, to PVH.

As far as I'm concerned, the size of the patch itself is immaterial; the 
only important question, regarding how to break down patches (just like 
in breaking down functions), is how easy or difficult it is to 
understand the whole thing.

Now it's typically the case that long patches are hard to understand, 
and that breaking them down into smaller chunks makes them easier to 
read.  But a division like this, where you've moved some random hunk 
into a different patch with which it has no logical relation, makes the 
series *harder* to understand, not easier.

Additionally, as the series evolves, it makes it difficult to keep all 
of the dependencies straight.  Suppose you changed your approach for 
that future patch so that you didn't need this public anymore. You, and 
all the reviewers, could easily forget about the dependency, since it's 
in a separate patch which may have already been classified as "OK".

It's like taking a function named foo() and breaking it down into 
foo_1() and foo_2().  You're making the function shorter, but achieving 
the opposite of what having short functions is supposed to achieve. :-)

>
>>> +    if ( (rc = raw_copy_from_guest(sig, (char *)eip,
>>> sizeof(sig))) != 0 ) {
>>>           propagate_page_fault(eip + sizeof(sig) - rc, 0);
>>>           return EXCRET_fault_fixed;
>>> @@ -931,7 +936,7 @@ static int emulate_forced_invalid_op(struct
>>> cpu_user_regs *regs) eip += sizeof(sig);
>>>
>>>       /* We only emulate CPUID. */
>>> -    if ( ( rc = copy_from_user(instr, (char *)eip,
>>> sizeof(instr))) != 0 )
>>> +    if ( ( rc = raw_copy_from_guest(instr, (char *)eip,
>>> sizeof(instr))) != 0 ) {
>>>           propagate_page_fault(eip + sizeof(instr) - rc, 0);
>>>           return EXCRET_fault_fixed;
>>> @@ -1076,6 +1081,12 @@ void propagate_page_fault(unsigned long
>>> addr, u16 error_code) struct vcpu *v = current;
>>>       struct trap_bounce *tb = &v->arch.pv_vcpu.trap_bounce;
>>>
>>> +    if ( is_pvh_vcpu(v) )
>>> +    {
>>> +        hvm_inject_page_fault(error_code, addr);
>>> +        return;
>>> +    }
>> Would it make more sense to rename this function
>> "pv_inject_page_fault", and then make a macro to switch between the
>> two?
> I don't think so, propagate_page_fault seems generic enough.

What I meant was something similar to what I suggested for patch 10 -- 
make propagate_page_fault() truly generic, by making it check what mode 
is running and calling either pv_inject_page_fault() or 
hvm_inject_page_fault() as appropriate.

  -George