Re: [PATCH v5 08/11] xen/arm: Enable the existing x86 virtual PCI support for ARM.

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.10.2021 21:27, Stefano Stabellini wrote:
> On Wed, 13 Oct 2021, Jan Beulich wrote:
>> On 13.10.2021 16:51, Oleksandr Andrushchenko wrote:
>>> On 13.10.21 16:00, Jan Beulich wrote:
>>>> On 13.10.2021 10:45, Roger Pau Monné wrote:
>>>>> On Wed, Oct 06, 2021 at 06:40:34PM +0100, Rahul Singh wrote:
>>>>>> --- /dev/null
>>>>>> +++ b/xen/arch/arm/vpci.c
>>>>>> @@ -0,0 +1,102 @@
>>>>>> +/*
>>>>>> + * xen/arch/arm/vpci.c
>>>>>> + *
>>>>>> + * This program is free software; you can redistribute it and/or modify
>>>>>> + * it under the terms of the GNU General Public License as published by
>>>>>> + * the Free Software Foundation; either version 2 of the License, or
>>>>>> + * (at your option) any later version.
>>>>>> + *
>>>>>> + * This program is distributed in the hope that it will be useful,
>>>>>> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
>>>>>> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
>>>>>> + * GNU General Public License for more details.
>>>>>> + */
>>>>>> +#include <xen/sched.h>
>>>>>> +
>>>>>> +#include <asm/mmio.h>
>>>>>> +
>>>>>> +#define REGISTER_OFFSET(addr)  ( (addr) & 0x00000fff)
>>>>>> +
>>>>>> +/* Do some sanity checks. */
>>>>>> +static bool vpci_mmio_access_allowed(unsigned int reg, unsigned int len)
>>>>>> +{
>>>>>> +    /* Check access size. */
>>>>>> +    if ( len > 8 )
>>>>>> +        return false;
>>>>>> +
>>>>>> +    /* Check that access is size aligned. */
>>>>>> +    if ( (reg & (len - 1)) )
>>>>>> +        return false;
>>>>>> +
>>>>>> +    return true;
>>>>>> +}
>>>>>> +
>>>>>> +static int vpci_mmio_read(struct vcpu *v, mmio_info_t *info,
>>>>>> +                          register_t *r, void *p)
>>>>>> +{
>>>>>> +    unsigned int reg;
>>>>>> +    pci_sbdf_t sbdf;
>>>>>> +    unsigned long data = ~0UL;
>>>>>> +    unsigned int size = 1U << info->dabt.size;
>>>>>> +
>>>>>> +    sbdf.sbdf = MMCFG_BDF(info->gpa);
>>>>>> +    reg = REGISTER_OFFSET(info->gpa);
>>>>>> +
>>>>>> +    if ( !vpci_mmio_access_allowed(reg, size) )
>>>>>> +        return 0;
>>>>>> +
>>>>>> +    data = vpci_read(sbdf, reg, min(4u, size));
>>>>>> +    if ( size == 8 )
>>>>>> +        data |= (uint64_t)vpci_read(sbdf, reg + 4, 4) << 32;
>>>>>> +
>>>>>> +    *r = data;
>>>>>> +
>>>>>> +    return 1;
>>>>>> +}
>>>>>> +
>>>>>> +static int vpci_mmio_write(struct vcpu *v, mmio_info_t *info,
>>>>>> +                           register_t r, void *p)
>>>>>> +{
>>>>>> +    unsigned int reg;
>>>>>> +    pci_sbdf_t sbdf;
>>>>>> +    unsigned long data = r;
>>>>>> +    unsigned int size = 1U << info->dabt.size;
>>>>>> +
>>>>>> +    sbdf.sbdf = MMCFG_BDF(info->gpa);
>>>>>> +    reg = REGISTER_OFFSET(info->gpa);
>>>>>> +
>>>>>> +    if ( !vpci_mmio_access_allowed(reg, size) )
>>>>>> +        return 0;
>>>>>> +
>>>>>> +    vpci_write(sbdf, reg, min(4u, size), data);
>>>>>> +    if ( size == 8 )
>>>>>> +        vpci_write(sbdf, reg + 4, 4, data >> 32);
>>>>> I think those two helpers (and vpci_mmio_access_allowed) are very
>>>>> similar to the existing x86 ones (see vpci_mmcfg_{read,write}), up to
>>>>> the point where I would consider moving the shared code to vpci.c as
>>>>> vpci_ecam_{read,write} and call them from the arch specific trap
>>>>> handlers.
>>>> Except that please can we stick to mcfg or mmcfg instead of ecam
>>>> in names, as that's how the thing has been named in Xen from its
>>>> introduction? I've just grep-ed the code base (case insensitively)
>>>> and found no mention of ECAM. There are only a few "became".
>>> I do understand that this is historically that we do not have ECAM in Xen,
>>> but PCI is not about Xen. Thus, I think it is also acceptable to use
>>> a commonly known ECAM for the code that works with ECAM.
>>
>> ACPI, afaik, also doesn't call this ECAM. That's where MCFG / MMCFG
>> actually come from, I believe.
> 
> My understanding is that "MCFG" is the name of the ACPI table that
> describes the PCI config space [1]. The underlying PCI standard for the
> memory mapped layout of the PCI config space is called ECAM. Here, it
> makes sense to call it ECAM as it is firmware independent.
> 
> [1] https://wiki.osdev.org/PCI_Express

Okay, I can accept this, but then I'd expect all existing uses of
MCFG / MMCFG in the code which aren't directly ACPI-related to get
replaced. Otherwise it's needlessly harder to identify that one
piece of code relates to certain other pieces.

Jan