Re: [Xen-devel] [PATCH 4/9] x86/pv: Implement pv_hypercall() in C

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 02/08/16 14:12, Jan Beulich wrote:
>>>> On 18.07.16 at 11:51, <andrew.cooper3@xxxxxxxxxx> wrote:
>> +long pv_hypercall(struct cpu_user_regs *regs)
>> +{
>> +    struct vcpu *curr = current;
>> +#ifndef NDEBUG
>> +    unsigned long old_rip = regs->rip;
>> +#endif
>> +    long ret;
>> +    uint32_t eax = regs->eax;
>> +
>> +    ASSERT(curr->arch.flags & TF_kernel_mode);
> I'm afraid TF_kernel_mode can't be relied on for 32-bit guests, so
> this needs to move into the if() below.

In which case it should become ASSERT(guest_mode_kernel(curr, regs))

>
>> +    if ( (eax >= NR_hypercalls) || !hypercall_table[eax] )
>> +         return -ENOSYS;
>> +
>> +    if ( !is_pv_32bit_vcpu(curr) )
>> +    {
>> +        unsigned long rdi = regs->rdi;
>> +        unsigned long rsi = regs->rsi;
>> +        unsigned long rdx = regs->rdx;
>> +        unsigned long r10 = regs->r10;
>> +        unsigned long r8 = regs->r8;
>> +        unsigned long r9 = regs->r9;
>> +
>> +#ifndef NDEBUG
>> +        /* Deliberately corrupt parameter regs not used by this hypercall. 
>> */
>> +        switch ( hypercall_args_table[eax] )
>> +        {
>> +        case 0: rdi = 0xdeadbeefdeadf00dUL;
>> +        case 1: rsi = 0xdeadbeefdeadf00dUL;
>> +        case 2: rdx = 0xdeadbeefdeadf00dUL;
>> +        case 3: r10 = 0xdeadbeefdeadf00dUL;
>> +        case 4: r8 = 0xdeadbeefdeadf00dUL;
>> +        case 5: r9 = 0xdeadbeefdeadf00dUL;
> Without comments, aren't these going to become 5 new Coverity
> issues?

There are no current warnings from the HVM side, so I doubt it. 
Coverities' logic is rather complicated, but in this case I think the
lack of any break statements at all is a sufficient hint that its fine.

>
>> --- a/xen/arch/x86/x86_64/compat/entry.S
>> +++ b/xen/arch/x86/x86_64/compat/entry.S
>> @@ -25,70 +25,10 @@ UNLIKELY_START(ne, msi_check)
>>          LOAD_C_CLOBBERED compat=1 ax=0
>>  UNLIKELY_END(msi_check)
>>  
>> -        movl  UREGS_rax(%rsp),%eax
>>          GET_CURRENT(bx)
>>  
>> -        cmpl  $NR_hypercalls,%eax
>> -        jae   compat_bad_hypercall
>> -#ifndef NDEBUG
>> -        /* Deliberately corrupt parameter regs not used by this hypercall. 
>> */
>> -        pushq UREGS_rbx(%rsp); pushq %rcx; pushq %rdx; pushq %rsi; pushq 
>> %rdi
>> -        pushq UREGS_rbp+5*8(%rsp)
>> -        leaq  compat_hypercall_args_table(%rip),%r10
>> -        movl  $6,%ecx
>> -        subb  (%r10,%rax,1),%cl
>> -        movq  %rsp,%rdi
>> -        movl  $0xDEADBEEF,%eax
>> -        rep   stosq
>> -        popq  %r8 ; popq  %r9 ; xchgl %r8d,%r9d /* Args 5&6: zero extend */
>> -        popq  %rdx; popq  %rcx; xchgl %edx,%ecx /* Args 3&4: zero extend */
>> -        popq  %rdi; popq  %rsi; xchgl %edi,%esi /* Args 1&2: zero extend */
>> -        movl  UREGS_rax(%rsp),%eax
>> -        pushq %rax
>> -        pushq UREGS_rip+8(%rsp)
>> -#define SHADOW_BYTES 16 /* Shadow EIP + shadow hypercall # */
>> -#else
>> -        /* Relocate argument registers and zero-extend to 64 bits. */
>> -        xchgl %ecx,%esi              /* Arg 2, Arg 4 */
>> -        movl  %edx,%edx              /* Arg 3        */
>> -        movl  %edi,%r8d              /* Arg 5        */
>> -        movl  %ebp,%r9d              /* Arg 6        */
>> -        movl  UREGS_rbx(%rsp),%edi   /* Arg 1        */
>> -#define SHADOW_BYTES 0  /* No on-stack shadow state */
>> -#endif
>> -        cmpb  $0,tb_init_done(%rip)
>> -UNLIKELY_START(ne, compat_trace)
>> -        call  __trace_hypercall_entry
>> -        /* Restore the registers that __trace_hypercall_entry clobbered. */
>> -        movl  UREGS_rax+SHADOW_BYTES(%rsp),%eax   /* Hypercall #  */
>> -        movl  UREGS_rbx+SHADOW_BYTES(%rsp),%edi   /* Arg 1        */
>> -        movl  UREGS_rcx+SHADOW_BYTES(%rsp),%esi   /* Arg 2        */
>> -        movl  UREGS_rdx+SHADOW_BYTES(%rsp),%edx   /* Arg 3        */
>> -        movl  UREGS_rsi+SHADOW_BYTES(%rsp),%ecx   /* Arg 4        */
>> -        movl  UREGS_rdi+SHADOW_BYTES(%rsp),%r8d   /* Arg 5        */
>> -        movl  UREGS_rbp+SHADOW_BYTES(%rsp),%r9d   /* Arg 6        */
>> -#undef SHADOW_BYTES
>> -UNLIKELY_END(compat_trace)
>> -        leaq  compat_hypercall_table(%rip),%r10
>> -        PERFC_INCR(hypercalls, %rax, %rbx)
>> -        callq *(%r10,%rax,8)
>> -#ifndef NDEBUG
>> -        /* Deliberately corrupt parameter regs used by this hypercall. */
>> -        popq  %r10         # Shadow RIP
>> -        cmpq  %r10,UREGS_rip+8(%rsp)
>> -        popq  %rcx         # Shadow hypercall index
>> -        jne   compat_skip_clobber /* If RIP has changed then don't clobber. 
>> */
>> -        leaq  compat_hypercall_args_table(%rip),%r10
>> -        movb  (%r10,%rcx,1),%cl
>> -        movl  $0xDEADBEEF,%r10d
>> -        testb %cl,%cl; jz compat_skip_clobber; movl %r10d,UREGS_rbx(%rsp)
>> -        cmpb  $2, %cl; jb compat_skip_clobber; movl %r10d,UREGS_rcx(%rsp)
>> -        cmpb  $3, %cl; jb compat_skip_clobber; movl %r10d,UREGS_rdx(%rsp)
>> -        cmpb  $4, %cl; jb compat_skip_clobber; movl %r10d,UREGS_rsi(%rsp)
>> -        cmpb  $5, %cl; jb compat_skip_clobber; movl %r10d,UREGS_rdi(%rsp)
>> -        cmpb  $6, %cl; jb compat_skip_clobber; movl %r10d,UREGS_rbp(%rsp)
>> -compat_skip_clobber:
>> -#endif
>> +        mov   %rsp, %rdi
>> +        call  pv_hypercall
>>          movl  %eax,UREGS_rax(%rsp)       # save the return value
> To follow the HVM model, this should also move into C.

I can do for now.

I haven't quite decided yet whether it would be sensible simplify the
continuation logic in the tail end of C, or by using -ERESTART at this
point.

~Andrew

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