Re: [PATCH v6 1/3] xen/vpci: Move ecam access functions to common code

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.10.2021 09:37, Bertrand Marquis wrote:
>> On 15 Oct 2021, at 07:29, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>> On 14.10.2021 19:09, Bertrand Marquis wrote:
>>>> On 14 Oct 2021, at 17:06, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>> On 14.10.2021 16:49, Bertrand Marquis wrote:
>>>>> @@ -305,7 +291,7 @@ static int vpci_portio_read(const struct 
>>>>> hvm_io_handler *handler,
>>>>>
>>>>>    reg = hvm_pci_decode_addr(cf8, addr, &sbdf);
>>>>>
>>>>> -    if ( !vpci_access_allowed(reg, size) )
>>>>> +    if ( !vpci_ecam_access_allowed(reg, size) )
>>>>>        return X86EMUL_OKAY;
>>>>>
>>>>>    *data = vpci_read(sbdf, reg, size);
>>>>> @@ -335,7 +321,7 @@ static int vpci_portio_write(const struct 
>>>>> hvm_io_handler *handler,
>>>>>
>>>>>    reg = hvm_pci_decode_addr(cf8, addr, &sbdf);
>>>>>
>>>>> -    if ( !vpci_access_allowed(reg, size) )
>>>>> +    if ( !vpci_ecam_access_allowed(reg, size) )
>>>>>        return X86EMUL_OKAY;
>>>>>
>>>>>    vpci_write(sbdf, reg, size, data);
>>>>
>>>> Why would port I/O functions call an ECAM helper? And in how far is
>>>> that helper actually ECAM-specific?
>>>
>>> The function was global before.
>>
>> I'm not objecting to the function being global, but to the "ecam" in
>> its name.
> 
> Adding ecam in the name was a request from Roger.
> This is just a consequence of this.

Roger - did you have in mind the uses here when asking for the addition
of "ecam"?

> One suggestion here could be to turn vpci_ecam_access_allowed into
> vpci_access_allowed

That's what I'm asking for.

> and maybe put this into vpci.h as a static inline ?

I'm not overly fussed here.

>>>>> @@ -434,25 +420,8 @@ static int vpci_mmcfg_read(struct vcpu *v, unsigned 
>>>>> long addr,
>>>>>    reg = vpci_mmcfg_decode_addr(mmcfg, addr, &sbdf);
>>>>>    read_unlock(&d->arch.hvm.mmcfg_lock);
>>>>>
>>>>> -    if ( !vpci_access_allowed(reg, len) ||
>>>>> -         (reg + len) > PCI_CFG_SPACE_EXP_SIZE )
>>>>> -        return X86EMUL_OKAY;
>>>>
>>>> While I assume this earlier behavior is the reason for ...
>>>
>>> Yes :-)
>>>
>>>>
>>>>> -    /*
>>>>> -     * According to the PCIe 3.1A specification:
>>>>> -     *  - Configuration Reads and Writes must usually be DWORD or smaller
>>>>> -     *    in size.
>>>>> -     *  - Because Root Complex implementations are not required to 
>>>>> support
>>>>> -     *    accesses to a RCRB that cross DW boundaries [...] software
>>>>> -     *    should take care not to cause the generation of such accesses
>>>>> -     *    when accessing a RCRB unless the Root Complex will support the
>>>>> -     *    access.
>>>>> -     *  Xen however supports 8byte accesses by splitting them into two
>>>>> -     *  4byte accesses.
>>>>> -     */
>>>>> -    *data = vpci_read(sbdf, reg, min(4u, len));
>>>>> -    if ( len == 8 )
>>>>> -        *data |= (uint64_t)vpci_read(sbdf, reg + 4, 4) << 32;
>>>>> +    /* Ignore return code */
>>>>> +    vpci_ecam_mmio_read(sbdf, reg, len, data);
>>>>
>>>> ... the commented-upon ignoring of the return value, I don't think
>>>> that's a good way to deal with things anymore. Instead I think
>>>> *data should be written to ~0 upon failure, unless it is intended
>>>> for vpci_ecam_mmio_read() to take care of that case (in which case
>>>> I'm not sure I would see why it needs to return an error indicator
>>>> in the first place).
>>>
>>> I am not sure in the first place why this is actually ignored and just
>>> returning a -1 value.
>>> If an access is not right, an exception should be generated to the
>>> Guest instead.
>>
>> No. That's also not what happens on bare metal, at least not on x86.
>> Faults cannot be raised for reasons outside of the CPU; such errors
>> (if these are errors in the first place) need to be dealt with
>> differently. Signaling an error on the PCI bus would be possible,
>> but would leave open how that's actually to be dealt with. Instead
>> bad reads return all ones, while bad writes simply get dropped.
> 
> So that behaviour is kept here on x86 and I think as the function is
> generic it is right for it to return an error here. It is up to the caller to
> ignore it or not.
> To make this more generic I could return 0 on success and -EACCESS,
> the caller would then handle it as he wants.

I think boolean is sufficient here, but I wouldn't object to errno-
style return values. All I do object to is int when boolean is meant.

>>>>> +int vpci_ecam_mmio_write(pci_sbdf_t sbdf, unsigned int reg, unsigned int 
>>>>> len,
>>>>> +                         unsigned long data)
>>>>> +{
>>>>> +    if ( !vpci_ecam_access_allowed(reg, len) ||
>>>>> +         (reg + len) > PCI_CFG_SPACE_EXP_SIZE )
>>>>> +        return 0;
>>>>> +
>>>>> +    vpci_write(sbdf, reg, min(4u, len), data);
>>>>> +    if ( len == 8 )
>>>>> +        vpci_write(sbdf, reg + 4, 4, data >> 32);
>>>>> +
>>>>> +    return 1;
>>>>> +}
>>>>> +
>>>>> +int vpci_ecam_mmio_read(pci_sbdf_t sbdf, unsigned int reg, unsigned int 
>>>>> len,
>>>>> +                        unsigned long *data)
>>>>> +{
>>>>> +    if ( !vpci_ecam_access_allowed(reg, len) ||
>>>>> +         (reg + len) > PCI_CFG_SPACE_EXP_SIZE )
>>>>> +        return 0;
>>>>> +
>>>>> +    /*
>>>>> +     * According to the PCIe 3.1A specification:
>>>>> +     *  - Configuration Reads and Writes must usually be DWORD or smaller
>>>>> +     *    in size.
>>>>> +     *  - Because Root Complex implementations are not required to 
>>>>> support
>>>>> +     *    accesses to a RCRB that cross DW boundaries [...] software
>>>>> +     *    should take care not to cause the generation of such accesses
>>>>> +     *    when accessing a RCRB unless the Root Complex will support the
>>>>> +     *    access.
>>>>> +     *  Xen however supports 8byte accesses by splitting them into two
>>>>> +     *  4byte accesses.
>>>>> +     */
>>>>> +    *data = vpci_read(sbdf, reg, min(4u, len));
>>>>> +    if ( len == 8 )
>>>>> +        *data |= (uint64_t)vpci_read(sbdf, reg + 4, 4) << 32;
>>>>> +
>>>>> +    return 1;
>>>>> +}
>>>>
>>>> Why do these two functions return int/0/1 instead of
>>>> bool/false/true (assuming, as per above, that them returning non-
>>>> void is warranted at all)?
>>>
>>> This is what the mmio handlers should return to say that an access
>>> was ok or not so the function stick to this standard.
>>
>> Sticking to this would be okay if the functions here needed their
>> address taken, such that they can be installed as hooks for a
>> more general framework to invoke. The functions, however, only get
>> called directly. Hence there's no reason to mirror what is in need
>> of cleaning up elsewhere. I'm sure you're aware there we're in the
>> (slow going) process of improving which types get used where.
>> While the functions you refer to may not have undergone such
>> cleanup yet, we generally expect new code to conform to the new
>> model.
> 
> I am ok to rename those to vpci_ecam_{read/write}.
> Is it what you want ?

Yes, that's what I've been asking for, and I just saw Roger requesting
the same. (I'm a little puzzled about the context though, as you reply
looks disconnected here.)

Jan