Re: [PATCH v3 3/3] x86/PVH: Support relocatable dom0 kernels

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.03.2024 14:45, Jason Andryuk wrote:
> On 2024-03-20 10:39, Jan Beulich wrote:
>> On 19.03.2024 21:58, Jason Andryuk wrote:
>>> +/* Find an e820 RAM region that fits the kernel at a suitable alignment. */
>>> +static paddr_t __init find_kernel_memory(
>>> +    const struct domain *d, struct elf_binary *elf,
>>> +    const struct elf_dom_parms *parms)
>>> +{
>>> +    paddr_t kernel_size = elf->dest_size;
>>> +    unsigned int i;
>>> +
>>> +    for ( i = 0; i < d->arch.nr_e820; i++ )
>>> +    {
>>> +        paddr_t start = d->arch.e820[i].addr;
>>> +        paddr_t end = d->arch.e820[i].addr + d->arch.e820[i].size;
>>> +        paddr_t kstart, kend;
>>> +
>>> +        if ( d->arch.e820[i].type != E820_RAM ||
>>> +             d->arch.e820[i].size < kernel_size )
>>> +            continue;
>>> +
>>> +        kstart = ROUNDUP(start, parms->phys_align);
>>> +        kstart = max_t(paddr_t, kstart, parms->phys_min);
>>
>> I'd generally try to avoid max_t(), but I cannot think of any good way
>> of expressing this without using it.
>>
>>> +        kend = kstart + kernel_size;
>>> +
>>> +        if ( kend > parms->phys_max )
>>
>> So despite its default phys_max is exclusive aiui. Otherwise with
>> kend being exclusive too, this would look to be off by one.
> 
> Yes, I'll fix the off-by-one.  Hmmm, phys_max being 32bit, we want it to 
> be inclusive to represent the maximum range.

I specifically didn't ask for that, as I think it would end up a little
awkward. But I also don't mind you adjusting things to that effect.

>>> +            return 0;
>>> +
>>> +        if ( kend <= end )
>>> +            return kstart;
>>> +    }
>>> +
>>> +    return 0;
>>> +}
>>> +
>>> +/* Check the kernel load address, and adjust if necessary and possible. */
>>> +static bool __init check_and_adjust_load_address(
>>> +    const struct domain *d, struct elf_binary *elf, struct elf_dom_parms 
>>> *parms)
>>> +{
>>> +    paddr_t reloc_base;
>>> +
>>> +    if ( check_load_address(d, elf) )
>>> +        return true;
>>> +
>>> +    if ( !parms->phys_reloc )
>>> +    {
>>> +        printk("%pd kernel: Address conflict and not relocatable\n", d);
>>> +        return false;
>>
>> This better wouldn't result in -ENOMEM in the caller. Then again reasons
>> are logged, so the specific error code perhaos doesn't matter that much.
> 
> Failure here is turned into -ENOMEM by pvh_load_kernel().   -ENOMEM is 
> already returned for later failure with find_memory(), so I thought it 
> was acceptable.  Without this code, elf_load_binary() would failed with 
> -1 and that would be returned.  I'll change it to whatever you prefer.

ENOSPC would likely make it easily distinguishable from anything else.

>>> +    }
>>> +
>>> +    reloc_base = find_kernel_memory(d, elf, parms);
>>> +    if ( reloc_base == 0 )
>>
>> With ! used above please be consistent and do so here, too.
> 
> phys_reloc is a bool, and reloc_base is a paddr_t.

Hmm, I see. But still - we generally prefer ! over "== 0" (or even "== NULL").

>>> +    {
>>> +        printk("%pd kernel: Failed find a load address\n", d);
>>> +        return false;
>>> +    }
>>> +
>>> +    if ( opt_dom0_verbose )
>>> +        printk("%pd kernel: Moving [%p, %p] -> [%"PRIpaddr", 
>>> %"PRIpaddr"]\n", d,
>>> +               elf->dest_base, elf->dest_base + elf->dest_size - 1,
>>> +               reloc_base, reloc_base + elf->dest_size - 1);
>>> +
>>> +    parms->phys_entry = reloc_base +
>>> +                            (parms->phys_entry - (paddr_t)elf->dest_base);
>>> +    elf->dest_base = (char *)reloc_base;
>>
>> Just as a remark, no request to change anything: We're not dealing with
>> strings here. Hence char * isn't quite right, just that "dest_base" is
>> of that type (for whatever reason).
> 
> I think the reason is just to be byte addressable.

Sure, but that would call for void *, unsigned char *, or uint8_t *.

Jan