Re: [PATCH RFC 3/6] x86/ioapic: RTE modifications must use ioapic_write_entry

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.04.2022 15:21, Roger Pau Monne wrote:
> Do not allow to write to RTE registers using io_apic_write and instead
> require changes to RTE to be performed using ioapic_write_entry.

Hmm, this doubles the number of MMIO access in affected code fragments.

> --- a/xen/arch/x86/include/asm/io_apic.h
> +++ b/xen/arch/x86/include/asm/io_apic.h
> @@ -161,22 +161,11 @@ static inline void __io_apic_write(unsigned int apic, 
> unsigned int reg, unsigned
>  
>  static inline void io_apic_write(unsigned int apic, unsigned int reg, 
> unsigned int value)
>  {
> -    if ( ioapic_reg_remapped(reg) )
> -        return iommu_update_ire_from_apic(apic, reg, value);
> +    /* RTE writes must use ioapic_write_entry. */
> +    BUG_ON(reg >= 0x10);
>      __io_apic_write(apic, reg, value);
>  }
>  
> -/*
> - * Re-write a value: to be used for read-modify-write
> - * cycles where the read already set up the index register.
> - */
> -static inline void io_apic_modify(unsigned int apic, unsigned int reg, 
> unsigned int value)
> -{
> -    if ( ioapic_reg_remapped(reg) )
> -        return iommu_update_ire_from_apic(apic, reg, value);
> -    *(IO_APIC_BASE(apic) + 4) = value;
> -}

While the last caller goes away, I don't think this strictly needs to
be dropped (but could just gain a BUG_ON() like you do a few lines up)?

Jan