RE: [PATCH] x86: Implement Linear Address Masking support

[David Laight]

From: Kirill A. Shutemov
> Sent: 11 May 2022 03:28
> 
> Linear Address Masking feature makes CPU ignore some bits of the virtual
> address. These bits can be used to encode metadata.
> 
> The feature is enumerated with CPUID.(EAX=07H, ECX=01H):EAX.LAM[bit 26].
> 
> CR3.LAM_U57[bit 62] allows to encode 6 bits of metadata in bits 62:57 of
> user pointers.
> 
> CR3.LAM_U48[bit 61] allows to encode 15 bits of metadata in bits 62:48
> of user pointers.
> 
> CR4.LAM_SUP[bit 28] allows to encode metadata of supervisor pointers.
> If 5-level paging is in use, 6 bits of metadata can be encoded in 62:57.
> For 4-level paging, 15 bits of metadata can be encoded in bits 62:48.
> 
...
> +static vaddr clean_addr(CPUArchState *env, vaddr addr)
> +{
> +    CPUClass *cc = CPU_GET_CLASS(env_cpu(env));
> +
> +    if (cc->tcg_ops->do_clean_addr) {
> +        addr = cc->tcg_ops->do_clean_addr(env_cpu(env), addr);

The performance of a conditional indirect call will be horrid.
Over-engineered when there is only one possible function.

....
> +
> +static inline int64_t sign_extend64(uint64_t value, int index)
> +{
> +    int shift = 63 - index;
> +    return (int64_t)(value << shift) >> shift;
> +}

Shift of signed integers are UB.

> +vaddr x86_cpu_clean_addr(CPUState *cs, vaddr addr)
> +{
> +    CPUX86State *env = &X86_CPU(cs)->env;
> +    bool la57 = env->cr[4] & CR4_LA57_MASK;
> +
> +    if (addr >> 63) {
> +        if (env->cr[4] & CR4_LAM_SUP) {
> +            return sign_extend64(addr, la57 ? 56 : 47);
> +        }
> +    } else {
> +        if (env->cr[3] & CR3_LAM_U57) {
> +            return sign_extend64(addr, 56);
> +        } else if (env->cr[3] & CR3_LAM_U48) {
> +            return sign_extend64(addr, 47);
> +        }
> +    }

That is completely horrid.
Surely it can be just:
	if (addr && 1u << 63)
		return addr | env->address_mask;
	else
		return addr & ~env->address_mask;
Where 'address_mask' is 0x7ff....
although since you really want a big gap between valid user and
valid kernel addresses allowing masked kernel addresses adds
costs elsewhere.

I've no idea how often the address masking is required?
Hopefully almost never?

copy_to/from_user() (etc) need to be able to use user addresses
without having to mask them.

	David

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